The crosslink density (CLD) for polyurethane elastomeric networks based on hydroxyl terminated polybutadiene and isophorone-diisocyanate was theoretically calculated with alpha-model equations the employing the functionality distribution and extent of reaction as input parameters. The theoretical crosslink density (v(t)) was compared with the CLD values computed from stress strain data evaluated at various strain rates. The methods for the calculation of the CLD from stress strain data were based on the Mooney-Rivlin and Young's modulus approaches. Theoretical stress strain curves were generated on the basis of v(t) conforming to both phantom and affine model calculations. The experimental stress strain plots aligned more closely to the affine model line. The deviation of the experimentally derived stress-strain curves from the theoretical affine curve was probably due to the presence of temporarily trapped physical entanglements. From the stress-train data, the concentrations of true chemical crosslinks and physical entanglements were estimated individually. (C) 2010 Wiley Periodicals, Inc. J Appl Polym Sci 117: 920-925, 2010