When a solution containing a single reactant is subjected to kinetic analysis with a reagent giving rise to a pseudo-first-order reaction, non-linear regression analysis of the concentrationtime data yields a random scatter of the residuals around the best fit to the pseudo-first-order equation. If the same equation is used when a second reactant is also present, systematic errors arise and yield a deviation plot having a characteristic shape. If the amplitude of that plot is substantially larger than the random error of measurement, the presence of the reactant can be detected, and its concentration can then be evaluated by non-linear regression onto the equation that takes its presence into account. The amplitude passes through a maximum as the relative concentration of the second reactant increases, or as the ratio of the rate constants increases. For any given ratio of concentrations, detection of the second reactant is impossible unless the ratio of the rate constants lies within a certain range, which will be governed by the data-acquisition schedule employed. For the particular schedule assumed here, examination of these dependences shows, for example, that it should be possible to detect the second reactant if its concentration is 2.5 per cent of that of the first reactant and if the ratio of the rate constants is between 7.1 and 21.7.