The homogeneity range of U2Co17-xSix system with the hexagonal Th2Ni17-type crystal structure extends from x= 1 to 3.4. The variation of the magnetic properties within the homogeneity range was studied on single crystals. All the compounds are ferromagnetic, M-s, and T-C decrease monotonously with increasing Si content. The strongly modified magnetic anisotropy of U2Co17-xSix, as compared to isostructural Lu2Co17-xSix with the non-magnetic Lu, points to a magnetic state of U up to x = 3.0. The U contribution to K, decreases with increasing Si content and vanishes at x = 3.4 that can be treated as a transition from magnetic to non-magnetic state of U. Spin reorientation was observed with varying temperature in compounds with x <= 3 due to a competition of the U and Co sublattices anisotropies which occurs as two second-order phase transitions of the "plane-cone" and the "cone-axis" type. (c) 2006 Elsevier B.V All rights reserved.