The influence of temperature variations and uncorrect positioning of the cuvette in its holder on the accuracy of absorbance measurements was thoroughly discussed in the case of potassium dichromate and nicotinic acid solutions to be used as reference materials for the periodic control of spectrophotometers. Results showed that temperature control to within ± 0.5 °C is sufficient. On the other hand, the error connected with cell positioning was found to be negligible under the usual working conditions. Finally, the effect of the purity degree on the absorbance values of the above substances was investigated.