The influence of hydrostatic pressure up to 1.1 GPa on critical parameters (To, Hc~, j~) of YBa2Cu4Os (Y 124) and Gd-Ba2Cu4Os (Gd124) ceramics has been investigated using a CuBe pressure bomb. A linear increase of Tc with the growth of pressure was found, with d Tc/dp = 5.5 K/GPa and 4.5 K/GPa for Y124 and Gd 124, respectively. The difference in Tc increase on pressure, dTJdp, between the two compounds is assumed to have the following possible mechanisms: first, the replacement of yttrium by gadolinium constrains the change of interionic distances within and between the CuO2 planes and thus the change of charge transfer integral having an effect on the density of electronic states at CuO2 planes; secondly, this replacement modifies the charge transfer into the CuO2 planes from rare-earth sites.
It was found that the critical current density increased a little under pressure; nevertheless a different behaviour on temperature has been observed for the two compounds. Insignificant changes of Hot under pressure were found at temperatures up to 40 K. However, at higher temperatures a little growth of Hc~ was observed.