The effect of pressure on the ferroelectric-paraelectric phase transition in the guanidinium compound [C(NH2)(3)](4)Cl2SO4 (G(4)Cl(2)SO(4)) was investigated in the pressure p range up to 500 MPa. Its phase transition temperature T-c decreases with initial pressure coefficient of dT(c)/dp = -1.2 x 10(-1) degrees C/MPa, in contrast to the positive pressure coefficient of the ferroelastic phase transition temperature in G(2)PbI(4). The relative permittivity epsilon(r) in the paraelectric phase above T-c obeys the Curie-Weiss law under pressures p. The peak value of epsilon(r) at T-c, epsilon(p) decreases remarkably with increasing p. Such decrease of the peak value of epsilon(p) with p results from both the increase in the temperature difference Delta T between T-c and the Curie-Weiss temperature T-o and the decrease in Curie constant C. The thermal hysteresis in T-c becomes larger (11 and 23 degrees C at 0 and 300 MPa, respectively) under pressures. The phase transition from ferroelectric II to paraelectric I at T-c becomes sluggish and broad with increasing p. The T-c deviates from the linear relationship to the non-linear one in T-c with p and tends to the change in shape of its curve towards the concave curve, in contrast to the convex one in the KH2PO4 type hydrogen-bonded ferroelectric. (C) 2010 Elsevier B. V. All rights reserved.