We compare the thermal lens (TLS) and the optical transmission (OT) spectroscopy techniques to monitor the kinetic of a photocatalytic reaction. For this, an OT measurement facility was added to a TLS set-up. The TLS was implemented in a microspatial configuration named thermal lens microscopy (TLM). Methylene blue (MB) in Water solutions were used as test samples within a concentration range in which both techniques show good sensitivity. Within this range, the limit of detection obtained by TLM was about one order of magnitude smaller than that achieved by OT. The methylene blue concentration evolution with photocatalytic reaction time was measured with both techniques, showing a good agreement between their results. A ZnO thin film deposited on a glass substrate by the spray pyrolysis technique was used as catalyst, and the reaction was induced by UV-violet light.