I did not find how Mathematica calculate the AIC criteria in NonLinearModelFiT, but I deduced the formula from the calculated AIC as AIC=k+1 +n (Log[2 Pi RSS/(n-k)]+1) where k is the number of parameter of the fit +1. It seems to me that there is an error with the k+1 because the equation should be AIC=k +n (Log[2 Pi RSS/(n-k)]+1) The extra 1 will be eliminated when taking the AIC difference, but I would like to be able to compute the correct AIC value or to justify the Mathematica values. Does anyone have clues about Mathematica AIC calculation?