Photoconductance measurements are frequently used to determine the minority-carrier effective lifetime (tau(eff)), from which the bulk lifetime (tau(b)) and surface recombination velocity (S) must be extracted. The exact solution to the continuity equation is used to determine the conditions of validity for three approximate equations relating tau(eff) to tau(b) and S: the steady-state approximation, the transient approximation, and the widely used simple equation (1/tau(eff) = 1/tau(b) + 2S/W). We show that only the steady-state approximation matches the exact solution over the entire range Of tau(eff), when the lamp time constant is 2.3 ms. When tau(eff) > 20 mus, all the equations give approximately the same result. However, when tau(eff) < 10 mus, only the steady-state equation is valid, while the error in the transient and simple equations may exceed 100%. (C) 2002 Elsevier Science B.V. All rights reserved.