Thanks for your help. I've never used a mass matrix before and I have a couple of questions. First, how do I determine the index of my DAE (I thought it was only 1, but MATLAB is telling me it appears to be greater than 1)? Each equation contains only first derivatives, but they also contain other variables and first derivatives (e.g. dx3 = x3 + 4*x1 + 2*dx2). Since after looking at some information on indices of DAEs I'm pretty sure mine is greater than 1 and thus not solvable using MATLAB (or at least ODE15s), do you have any suggestions of programs that actually do this?