Aqueous solubilities of calix[6]arene and 4-tert-butylcalix[4]arene were measured at temperatures of (433 to 533) K and (433 to 513) K, respectively, along the 5 MPa isobar. The relative rates of increase in the aqueous solubility with temperature, (partial derivative In x(2)/partial derivative T)(P,sigma), for the two solutes differ markedly, with values of 0.042 K-1 for calix[6]arene and 0.080 K-1 for 4-tert-butylcalix[4]arene. The solubility (equilibrium mole fraction) of 4-tert-butylcalix[4]arene is always lower than that of calix[6]arene at the same temperature, possibly reflecting the presence of four bulky 4-tert-butyl groups in the former molecule and also the presence of only four hydroxyl groups in the former molecule compared with six in the latter. Best-fit coefficients of the modified Apelblat equation are also reported for the two calixarenes.