35C1 NQR measurements were carried out on complexes of pentachlorophenol with nitrogen bases as a function of pressure (up to 300 MPa) and of temperature. It is shown that the sign and magnitude of the pressure coefficient of the 35C1 NQR frequency is related to the degree of proton transfer. An anomaly in the pressure coefficient of v (35C1) has been observed near 50% of the proton transfer in the hydrogen bond. This anomaly is discussed assuming that the proton transfer equilibrium is pressure dependent. The fact that the transition from the double-well potential of the hydrogen bond to the single-well potential occurs in the critical manner is also taken into account.