Reaction diffusion has applications to a wide range of areas, including industrial, geological, and medical problems. An important aspect is the phenomenon of 'critical time', which is a measure of how long the reaction and diffusion processes take. In this work, an exact solution using a linear reaction function is used to explore three critical time definitions. The outcome of the analysis is a simple, novel relationship between the reaction and diffusion time scales. (c) 2011 Elsevier Ltd. All rights reserved.