(Solid + liquid) and (vapour + liquid) equilibria for the binary systems (butane + adamantane) and (butane + diamantane) were determined according to the synthetic method in the temperature range (320 to 530) K and the pressure range (0.3 to 10) MPa. For both systems also the three-phase curves (solid diamondoid + liquid + vapour) was determined. These curves show a pressure maximum and in the case of (butane + diamantane), the slope of this curve demonstrates a discontinuity, which can be explained by the existence of two solid modifications of diamantane. The influence of the occurrence of two modifications of diamantane can also be seen in isobaric (solid + liquid) equilibria. (C) 2003 Elsevier Science Ltd. All rights reserved.