Abstract The determination of pyridoxal in human serum was performed using a fluorimetric technique that is useful for the determination of compounds in samples with unknown background fluorescence, based on synchronous scans through a trajectory joining points of equal intensity of a fluorescence matrix three-dimensional spectrum. This technique, called matrix isopotential synchronous fluorescence can be improved by means of the application of derivatives. The determination of pyridoxal in human serum was performed with this technique without any prior separation steps. The validity, applicability and simplicity of the method were demonstrated. The measurements were performed in aqueous medium at pH 7.0 adjusted by adding 0.05 M phosphate buffer solution. A complete statisitical analysis of the experimental data was performed and the results showed that correlation coefficients are between 0.9901 and 0.9958 for all the calibration graphs, and in all cases the intercepts on the ordinate were negligible. The exprimental F (1.72) is smaller than theoretical value (3.89) as expected from an analysis of variance. The reproducibility of the method was tested and the results were a standard deviation of 0.0117 μg ml −1 and a relative error of 4.91%.