Immunoassay incubation is classically performed at elevated temperatures to speed up reaction processes. This acceleration has long been assumed to be due to an increased association constant between reactants. Using an interferometric method to visualize temperature differences inside a small reaction cuvet, we demonstrate that the temperature profiles inside the liquid cannot be caused by conduction but only by thermal convection. Numerical simulations further support this experimental evidence. This paper conclusively demonstrates that thermally induced mass transport occurs in immunoassay incubation. Current data show that earlier estimates of heat transport coefficients in such incubations have been probably overestimated by a factor 3.