We have studied the influence of columnar defects, created by heavy-ion (Kr) irradiation with doses up to 6 x 10(11) Kr-ions/cm(2), on the superconducting critical parameters of single crystalline (YxTm1-x)Ba2Cu3O7. Magnetisation measurements in pulsed fields up to 50 T in the temperature range 4.2-90 K revealed that: (i) in fields up to mu(0) H approximate to 20 T the critical current J(c)(H,T) is considerably enhanced and (ii) down to temperatures T similar to 40 K the irreversibility field H-irr(T) is strongly increased. The field range and magnitude of the J(c)(H,T) and H-irr(T) enhancement increase with increasing irradiation dose. To interpret these observations, an effective matching field B phi. was defined. Moreover, introducing columnar defects also changes the pinning force f(p) qualitatively. Due to stronger pinning of flux lines by the amorphous defects, the superconducting critical parameters largely exceed those associated with the defect structures in the unirradiated as-grown material: J(c,6x10)(11) (2)(Kr-ions / cm)(77 K, 5 T) greater than or equal to 10 J(c,ref) (77 K, 5 T). (C) 1999 Published by Elsevier Science B.V. All rights reserved.