Analysis of the acid content of coffee extracts has been performed by adaptation of a procedure, recently introduced by our laboratory, to the task of evaluating both the "volatile" and "non volatile" fractions. Besides useful changes in the preparation of the acid pool, diazobutane was used to obtain esters of volatile acids suitable for GC-MS analysis, to minimize losses of material by evaporation. The new procedures have been used to evaluate changes in the acid content of some Italian-style roasted coffee extracts after accelerated ageing (65 degrees C, 72 h) and in coffee beans after different amounts of roasting. The origin of formation of volatile acidity was also investigated. Electronic supplementary material is available if you access this article at http://dx.doi.org/10.1007/s00216-002-1561-y On that page (frame on the left side) a link takes you directly to the supplementary material.