A gauge has been devised to measure the average molecular weight for surface soft segments in poly[(dimethylsiloxane)-urea]s. This quantity, M(s), is defined analogously to the number-average molecular weight, M(n). Information on the weight fraction of each segment at the surface, which can be measured by techniques such as electron spectroscopy for chemical analysis, is required for the calculation of M(s). The use of this quantity is illustrated by calculating the M(s) for a series of copolymers containing soft segments of broad molecular weight distribution. The limitation of this method with respect to the average molecular weight of the soft segments and Possible remedies are discussed.