- A direct rheological technique was used to measure the critical time for gelation, t c , of a well characterized polydimethylsiloxane (PDMS) polymer network. This technique is based on the observation that the gel point of crosslinking systems occurs at the instant at which the viscoelastic behaviour reduces to power-law relaxation (critical state described by the gel equation). The extent of reaction, p(t) , was measured by a Fourier-transform infra-red method. The measured value of the critical extent of reaction, p c = p ( t c ) = 0.57 ± 0.01, agrees extremely well with the value predicted by mean-field theory. This agreement is an independent confirmation of the observation that power-law relaxation is a property of the polymer at the gel point.