P-31 Spin-rotation hyperfine splitting has been observed in the 1(01) <-- 0(00) transitions of CH2 = (PCl)-Cl-35 and CH2 = (PCl)-Cl-37 by pulsed-nozzle, FT microwave spectroscopy. The transient molecules CH2 = PCl were produced by using a pyrolysis nozzle and then isolated on the ca. 10-mu-s timescale.