The vibrational band shapes and their parameters of the C-H vibration of CH2I2 diluted with liquid CCl4 were measured as a function of composition. The linewidth increases from 8.7 cm-1 with decreasing mole fraction X of CH2I2 reaches a maximum value of 15.92 cm-1 near X=0.4 and decreases again in the limit of the diluted case. The frequency of the band center increases with decreasing X from 2965.37 to 2984.75 cm-1. These results are compared with the model of Knapp and Fischer for the concentration dependence of the vibrational linewidths and shifts. For the line shift, good agreement is found between theory and experiment by introducing a microscopic mole fraction. The model can be used to get an estimation on the difference between macroscopic and microscopic concentration. The predictions for the band shape are rather satisfactory in the whole concentration range. Deviations are due to the assumption that the band shapes in the model are Lorentzian for both neat liquids, whereas the experimental line shape in neat CH2I2 is not.