A simple dynamic batch inlet system (DBIS) has been developed for hydrocarbon compound analysis by mass spectrometry. The system consists of a small expansion unit (2 cm(3)) that is easily installed into the GC oven of commercial GC/MS systems. The main advantages of the dynamic batch inlet system compared to conventional all-glass heated inlet systems (AGHIS) are (1) simplicity of operation, (2) low cost, (3) ease of maintenance, (4) rapidity of analysis, (5) automation, and (6) small sample size. In this work, the operation of the DBIS is described and typical results obtained by the analysis of representative petroleum fractions are given.