This work shows the effect of curing time on the mechanical properties of smokeless fuel briquettes which were prepared with olive stone and a low-rank coal previously co-carbonised at 600 degreesC. Humates and molasses were used as binders which acts with different roles, as a film or matrix depending on the curing. Additives like H3PO4 were also tested. Green briquettes were cured at different times and the molecular changes were followed by both Fourier transform infrared spectroscopy (FT-IR) and temperature programmed decomposition (TPD) followed by mass spectrometry (MS). This last study helps to predict the final properties of the briquettes more clearly than infrared spectroscopy does. This technique appreciates changes when the impact-resistance test does not show differences. The effect of the H3PO4 on briquettes prepared with molasses produce a stabilisation of these materials. Moreover, the mechanical resistance of the studied briquettes is improved by the effect of the curing time. (C) 2002 Elsevier Science B.V. All rights reserved.