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I have a very complicated mathematica expression that I'd like to simplify by using a new, possibly dimensionless parameter. An example of my expression is: K=a*b*t/((t+f)c*d); (the actual expression is monstrously large, thousands of characters). I'd like to replace all occurrences of the expression t/(t+f) with p p=t/(t+f); The goal here is to find a replacement so that all t's and f's are replaced by p. In this case, the replacement p is a nondimensionalized parameter, so it seems like a good candidate replacement. I've not been able to figure out how to do this in mathematica (or if its possible). I tried: eq1= K==a*b*t/((t+f)c*d); eq2= p==t/(t+f); Solve[{eq1,eq2},K] Not surprisingly, this doesn't work. If there were a way to force it to solve for K in terms of p,a,b,c,d , this might work, but I can't figure out how to do that either. Thoughts? Edit #1 (11/10/11 - 1:30) [deleted to simplify] OK, new tact. I've taken p=ton/(ton+toff) and multiplied p by several expressions. I know that p can be completely eliminated. The new expression (in terms of p) is testEQ = A B p + A^2 B p^2 + (A+B)p^3; Then I made the substitution for p, and called (normal) FullSimplify, giving me this expression. testEQ2= (ton (B ton^2 + A^2 B ton (toff + ton) + A (ton^2 + B (toff + ton)^2)))/(toff + ton)^3; Finally, I tried all of the suggestions below, except the last (not sure how it works yet!) Only the eliminate option worked. So I guess I'll try this method from now on. Thank you. EQ1 = a1 == (ton (B ton^2 + A^2 B ton (toff + ton) + A (ton^2 + B (toff + ton)^2)))/(toff + ton)^3; EQ2 = P1 == ton/(ton + toff); Eliminate[{EQ1, EQ2}, {ton, toff}] A B P1 + A^2 B P1^2 + (A + B) P1^3 == a1 I should add, if the goal is

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