Relatively inexpensive and simple modifications to an AEI MS-902 double focusing high-resolution mass spectrometer allow chemical ionization studies to be undertaken without compromising the instrument's high resolving power or sensitivity. A standard AEI source block was made gas tight except for a 0.013 to 0.020 in. diameter ion exit and an 0.013-in. electron entrance hole. A specially designed direct insertion probe was utilized to introduce both the sample and reagent gas. The reagent gas itself was supplied via a system utilizing both the manufacturers cold inlet system and Matheson lecture bottles. An additional pumping system was added in parallel to the existing pumping system to handle the increased gas flow.