We consider the trapping reaction A + B → B in space dimension d = 1, where the A and B particles have diffusion constants D_A and D_B respectively. We calculate the probability, Q(t), that a given A particle has not yet reacted at time t. Exploiting a recent formulation in which the B particles are eliminated from the problem we find, for , where ρ is the density of B particles and C ∝ D_A/D_B for D_A/D_B ≪ 1.