We present the first measurement of the ambipolar diffusion length in mesoporous silicon using the steady state photocarrier grating technique. Taking also response time measurements into account, we can deduce a typical value of the ambipolar diffusion coefficient of about 1.8 x 10(-6) cm(2) s(-1). The mobility-lifetime products of majority and minority carriers are found to be (mu tau)(mai) = 10(-8) cm(2) V-1 and (mu tau)(min) = 7 x 10(-10) cm(2) V-1 respectively. Together with temperature-dependent dark conductivity and intensity-dependent secondary photocurrent measurements we argue that a high density of trap states with a broad energetic distribution is responsible for such a small diffusion coefficient.