This study examines the behaviour of nitrogen during coalification in two coal series: the sedimentologically homogeneous coal series of the Mahakam delta, Indonesia and a reference coal series composed of samples from several sources. The variations in the total nitrogen contents, in hydrolysable nitrogen, and in the natural isotopic abundance of 15N are reported. The behaviour of nitrogen during coalification presents three main stages: 1 . a first stage where microbial and/or chemical degradations produce a loss of nitrogen; 2. a second stage of preservation, where organic nitrogen does not take part in the reactions involved and consequently remains unchanged in the residual organic matter; and 3. a third stage of thermal alteration, where nitrogen is mainly released with methane.