This paper deals with some nonlinear problems which exponential and biexponential decays are involved in. A proof of the quasiconvexity of the error function in some of these problems of optimization is presented. This proof is restricted to fitting observations by means of exponentials having the form f (t) = λ 1 exp(kt) + λ 2. Based on this quasiconvexity, we propose an algorithm to estimate the best approximation to each of these decays. Besides, the robustness of this algorithm allows us to avoid initial guess.