Infrared methods for the analysis of the third monomer in ethylene–propylene–norbornenic diene terpolymers were developed. They are based either on the C[DOUBLE BOND]C stretching vibration near 6 μ or on the [DOUBLE BOND]CH[BOND] out-of-plane deformation vibration between 11 and 12 μ. Calibration was made by NMR on selected samples with high unsaturation content. Particular emphasis was laid on terpolymers with ethylidene norbornene, for which the reliability of the analysis for the third monomer was checked by several techniques, and the transferability to the determination of propylene of the calibration curves set up for the analysis of composition in ethylene–propylene copolymers was studied.