In standard English, we would not say "My salary has reduced by 10%." We would say it one of two ways: "My salary was reduced by 10%." "My salary has been reduced by 10%." This is because, even in the passive voice, it implies that there was an agent who was responsible (i.e., the company, the boss). The salary did not reduce itself. (If it somehow could, we then would say "My salary has reduced by 10%." But this is not possible, since a salary has no volition.)

Serena, there are so many contradictory point of views shown above and I am afraid you will get a bit confused. I like to add some comments to the previous: 1- The 2 verbs could be synonyms as stated in many dictionaries as well. The idea is that you asked for the difference between them,those cases where they can't be interchangeable. 2- In Wotzitoya's example : My salary has reduced by 10% or My salary has decreased by 10%. It is not correct to say my salary has reduced. Consider Erik's comment on that ,it is correct! 3- Stating previously that "Some of the examples the others provided could actually be used for both words perfectly fine" I consider that by using the word "some" he already implicitly means "not all",which is a contradiction with his previous statement :" Other than that, they are pretty much exactly the same". In my example for instance: "The company's total productivity decreased due to insufficient supply of row materials ." Here you can not replace " decreased " by "reduced"; you can only use it in the passive voice form " verb to be + reduced ". So it is not perfectly interchangeable here! 4- Although both verbs are the same in many applications ,they have sometimes distinct meanings that makes them not interchangeable. For example : "Reduce " means also to lower in grade or force somebody into an undignified situation. " He reduced them to a slavery ". Here you can't use the verb "decrease" instead. Another example: to "reduce" the strength of a solution . In this context you can't use " decrease" as well etc.... So the absolute statement of them being one and the same is not precisely correct. They are the same with distinction and obviously your question was about these "differences"!

'Reduce' is a verb only. The noun would be 'reduction'. Decrease is a verb and a noun too. Other than that, they are pretty much exactly the same. Some of the examples the others provided could actually be used for both words perfectly fine.

These two words do mean the same thing in general. However, "reduce usually refers to the action taken by an agent, while "decrease" usually refers to the drop in quantity itself. Therefore, "reduce" is usually used BEFORE the object and "decrease" is used AFTER the object: Example: "Tylenol will reduce fever." (used BEFORE "fever") "If the fever does not decrease within two hours, call your doctor." (used AFTER fever) "To reduce your risk of certain cancers, quit smoking." (used BEFORE "risk") "If you quit smoking, your risk for cancer will decrease." (used AFTER "risk") For this reason, you will see "reduce" preferred over "decrease" in most situations and phrases when the DOER is the focus (i.e., when placed before the object): REDUCE: "to reduce debt" "to reduce calories / caloric intake" "to reduce weight" (diet) "to reduce use" (of some natural resource or medication) "to reduce inflammation / fever / etc." (medical conditions) "to reduce risk / chances" "to reduce heat" (when cooking) "to reduce spending" "to reduce taxes / interest rates / etc." There are others. Also note: The noun form of "to reduce" is "reduction". Ex.: "a reduction in daily calorie intake" The noun form of "to decrease" remains "decrease". Ex.: "a pay decrease" Be careful with numbers and mathematics, where "reduce" and "decrease" do NOT mean the same thing. Here, " to decrease" means to subtract or make smaller, while "to reduce" means to state a number (usually a fraction) in the simplest way. Ex. Given the fraction 2/4 1. To decrease this might be to have only 1/3 or 1/4. 2. To reduce this would be to express it as 1/2 (the same amount as 2/4).

From my point of view i think there is no different "Decrease" sounds strange to me when used transitively. I use "reduce", rather than "decrease", when there is a direct object: "Reduce the number of working hours", etc.