Consider a one dimensional diffusion process on the diffusion interval I originated in x0 ∈ I. Let a(t) and b(t) be two continuous functions of t, t > t0 with bounded derivatives and with a(t) < b(t) and a(t), b(t) ∈ I, ∀t > t0. We study the joint distribution of the two random variables Ta and T b , first hitting times of the diffusion process through the two boundaries a(t) and b(t), respectively. We express the joint distribution of Ta, T b in terms of P (Ta < t, Ta < T b ) and P (T b < t, Ta > T b ) and we determine a system of integral equations verified by these last probabilities. We propose a numerical algorithm to solve this system and we prove its convergence properties. Examples and modeling motivation for this study are also discussed.