Single crystals of Mn2+-doped α-Sr2P2O7 have been studied by electron spin resonance at 35 Gc/sec. The space-group ambiguity has been resolved because the Mn2+ ions lie on a mirror plane, and therefore the unit cell must be centric. The spin-Hamilitonian parameters have been determined to be gx = 2.0021(10), gy = 2.0015(4), gz = 2.0012(4), b20 = − 177.1(10) G, b22 = − 118.5(46) G, Ax = − 91.9(10) G, Ay = − 92.8(13) G, and Az = − 90.8(7) G. The z principal axis lies in the ab plane 17° from the b crystallographic axis. Only one EPR active site has been found rather than the two anticipated on crystallographic grounds. No evidence of a crystallographic phase transformation was found in the temperature range 100°–300°K.