Infinite-dilution activity coefficients (gamma (infinity)(1)) are useful for the screening and selection of solvents for extractive distillation. A gas-liquid chromatography method has been used to acquire gamma (infinity)(1) data for 20 solutes in hexadecane and in n-methyl-2-pyrrolidone (NMP). The present measurements show good agreement with available literature data. However, out study indicates that the degree of solvent pre-saturation significantly affects the quality of the data obtained for NMP. We also assessed the UNIFAC group contribution model for predicting gamma (infinity)(1) for the hexadecane and NMP systems. The average absolute percent deviations for the 1987 UNIFAC model for NMP and hexadecane were 42 and 11, respectively. The errors for hexadecane were slightly lower for the 1993 UNIFAC model than for the 1987 model (8 and 11%, respectively). However, due to missing groups in the 1993 UNIFAC model, it could not be evaluated for the NMP systems. Functional groups needed for describing NMP were added to the 1993 UNLFAC model. Two cases were studied. In the first case, the entire NMP molecule was treated as one functional group. This yielded more accurate phase behavior predictions. In the second case, we added two functional groups, which could enhance the UNIFAC predictive abilities for other solvents containing these functional groups. The model parameters were optimized to fit data measured recently by our group. The overall average absolute deviations for the two cases were 12 and 20%, respectively.