After irradiation of a pentazadiene solution (in THF) in a cuvette by laser pulses, damped oscillations of the absorbance were observed and mistakenly interpreted as being of photochemical origin. Additional experiments gave results that were incompatible with our photochemical interpretation and led to the identification of convection as the physical mechanism behind the observed oscillations. A simulation model based on the Navier-Stokes equations reproduces the damped oscillations. Analytical formulas are given to estimate convection velocities in cuvettes and might help to prevent misinterpretation of photolysis experiments. They show that arbitrarily small horizontal temperature gradients lead to convection in even very thin cuvettes. In any standard temperature-controlled cuvette, the fluid has to be assumed to be in motion rather than at rest.