Although H-2 gas is used in coal mines as an important indicator to reflect the state of coal spontaneous combustion, the gas production of H-2 at low temperature has been scarcely reported in the literature. In this paper, the modes and release mechanism of molecular hydrogen were investigated for three different coal ranks below 200 degrees C. Batch reactor tests were performed in combination with chromatographic analysis of the coal oxidation process. The experimental results showed that molecular, hydrogen release mainly originated from coal oxidation rather than thermal decomposition of inherent hydrogen containing groups. The amount of hydrogen released increased with the coal rank. The H-2 release process during low temperature oxidation typically proceeds in two phases, namely H-2 slow release (T < 100 degrees C) and H-2 accelerated release (T > 100 degrees C) phases. Experiments with model compounds revealed aldehyde compounds to noticeably produce H-2. Coal plays a positive role in promoting the aldehyde groups to release H-2 and CO2, but an opposite trend was observed in the case of CO. As revealed by Fourier transform infrared (FTIR) spectroscopy, the amount of aliphatic structures significantly decreased with the oxidation intensity, and a drastic increase in the aldehyde content was found at temperatures above 120 degrees C. Additionally, the path for the formation of H-2 during low-temperature oxidation of coal was provided.