The Maillard reaction is one of the dominant processes forming humic substances in natural waters affecting environmental pollution. In this study, we succeeded in quantitatively following decreases of starting materials (glycine and ribose) of the Maillard reaction by in situ infrared (IR) transmission spectroscopic observations with an original heatable liquid cell. The obtained kinetic parameters extrapolated to a typical Earth's surface aquatic environments (15 degrees C) indicate the half-life of around 2 years for the decrease of these components as the first step for the formation of humic substances.