They are different, but in a way similar to 'collocation' - http://grammar.about.com/od/c/g/collocationterm.htm. Both are grammatically acceptable actually; as is "lenient to". Hopefully these examples will demonstrate: - "Judges need to stop being lenient on female crime" - "I should be more lenient with him" - "I was growing very lenient to my master" (from the famous book 'Jane Eyre') Each has a subtle difference in relation to its context. "Lenient on" often fits better with concepts/categories (e.g. types of crime), though it is often used for people ("lenient on myself", "lenient on my students" etc.) "Lenient with" fits better with relationships with people. "Police should be less lenient with speeding" sounds awkward. "Lenient to" is the least common (in my opinion!), and also tends to fit better for people. "Police should be less lenient to speeding" sounds awkward. You ask some great questions, HCast!

Yes, I agree with Theresa, the only correct one is "lenient with." I believe that the reason people make this mistake is that they confuse it the common phrase "go easy on me." That should only used in informal speech, but it has a similar meaning. The infinitives are 'to go easy on' and 'to be lenient with.'

Hi, They are definately different. The correct one is "lenient with" myself. The original source of lenient is Latin, and you cannot be "on" yourself. Trust me. Good luck and good question. Theresa