We will discuss quasi-likelihood methods for fitting adsorption isotherms to data obtained from Second Harmonic Generation (SHG) experiments carried out in the Department of Chemistry at the University of Southampton. Adsorption isotherms are nonlinear models which relate the properties of molecules in the surface layer to the concentration of the solution and SHG experiments involve the use of lasers to study surface layers. Interest is in estimating specific coefficients such as the equilibrium constant which relates to the surface behaviour and the cosine of the difference in the phase angle which relates to the way the data were collected. We will describe some of the background to the models and the data and then discuss several sets of data. Issues of convergence (or lack thereof) arose in nearly every case and much of the interest is in teasing out why this happens so that we can work out what to try next. This is very much an applied talk which is intended to be an accessible account of some practical experience with fitting nonlinear models.