The course of the activation of 3'-acetylthymidine 5'-phosphate by TPS and DCC were followed up by 31P FT nmr. The fact that the "metaphosphate" (delta-5.1) first becomes detectable only at later stage of the activation and does coexist with pyrophosphate and triphosphate suggests that the pyridinium derivative of "metaphosphate" is most probably not directly formed from the hypothetical mixed anhydride or active "pseudourea" right at the beginning of the reaction of pdTac with TPS or DCC, but rather formed at later stage of the activation reaction from the degradation of the pyro- and triphosphates by the activating agent. The mixed anhydride or the active "pseudourea" is most possibly the active key intermediate.