The sensitive time of a thin gap spark chamber immersed in a magnetic field was measured. The magnetic field was normal to the electric field of the chamber, and various amounts of Freon 12 (CCl2F2) were used to decrease the sensitive time. A concentration of 100 ppm Freon 12 was sufficient to make the sensitive time less than ½ &mgr;sec, with an efficiency of greater than 70% for delays of less than 300 nsec.