The activity of the pure beta-emitter phosphorus-33 ( 3 3 P) has been directly determined by the triple-to-double coincidence ratio (TDCR) efficiency calculation technique, thus extending the number of radionuclides that have been standardized by this simple, non-extrapolation liquid scintillation (LS) method. The major advantage of inherently accounting for chemical quenching without the need for additional external measurements is demonstrated.The 3 3 P solution activity concentration was verified by tracing with cobalt-60, utilizing 4π(LS)β-γ coincidence counting. Agreement to within 1.2% confirmed the reliability of the TDCR measurements.