We report in situ ion mass spectrometer measurements from the coma of Comet 19P/Borrelly, obtained during the flyby of the Deep Space 1 spacecraft on 22 September 2001. Cometary ions were detected at distances from ~5.5 ¿ 105 km to 2200 km from the nucleus of the comet. The predominant heavy ions observed during the seven minute interval about closest approach to Borrelly included OH+ (at 57% of the total water-group ion density), H2O+ (at 29%), O+ (at 13%), CH3+ (at 5%), and CH2+ (at 4%). Of particular note is the small amount of H3O+ (<9%), as this was the most abundant molecular ion observed at closest approach to Comet Halley. This difference is due to the difference in water production rates between the two comets.