IR transmission and Raman spectra of toluylene diisocyanate and hexamethylene diisocyanate and IR spectra of diphenylmethane diisocyanate, partially polymerized diphenylmethane diisocyanate, and polyisocyanurate were obtained. Toluylene diisocyanate and partially polymerized diphenylmethane diisocyanate form associates in the liquid state, which leads to anomalous intensity changes and a frequency shift of the nu(as)(NCO) band in the spectrum of the treated rubber surface with time. The associate formation influences positively the properties of diisocyanates as rubber-surface modifiers for adhesion.