Dear R-Help group, I'm trying to fit some smoothing spline models using the snr function in the ASSIST library. My model is the following: y~log(exp(a0-a1*x)+exp(b0-b1(x))) where b1(x) is a nonparametric function. It's basically a biexponential decay, with wiggliness in 'phase 2' decay. I can easily fit this using regression splines, but I'd like to see how smoothing splines work in this scenario. I tried to use the snr function, as follows snr(y~log(exp(a0-a1*x)+exp(b0-b1(x))),func=list(b1(u)~list(~u,cubic2 (u))),params=list(a0+a1+b0~1),data=mydata,start=list(params=mystart)) This runs happily, but interpolates the data, rather than smoothing it. This is probably due to some identifiability issues, but I'm not sure how to parameterize the model to fix this. Any suggestions? Best wishes Simon