The yields of chemical reactions may vary considerably with parameters such as solvent, catalyst, temperature and reaction time. Here we have monitored the metallation of a porphyrin with zinc as a function of solvent. The porphyrin starting material and product are both highly coloured and the reaction proceeds with a colour change. Monitoring is ideally suited to the BMG LABTECH FLUOstar Omega microplate reader due to its ability to acquire a full UV-visible absorbance spectrum of each well in less than one second. This and other absorbance based assays can also be performed on BMG LABTECH's new absorbance microplate, cuvette and low-volume reader SPECTROstar Nano.