The structure and phase evolution of nanocrystalline Ce(1-x)Ln(x)O(2-x/2-delta) (Ln = Yb, Lu, x = 0 - 1) oxides upon heating in H(2) was studied for the first time. Up to 950 degrees C the samples were single-phase, with structure changing smoothly with x from fluorite type (F) to bixbyite type (C). For the Lu-doped samples heated at 1100 degrees C in the air and H(2), phase separation into coexisting F- and C-type structures was observed for ~0.40 < x < ~0.70 and ~0.25 < x < ~0.70, respectively. It was found also that addition of Lu(3+) and Yb(3+) strongly hinders the crystallite growth of ceria during heat treatment at 800 and 950 degrees C in both atmospheres. Valency of Ce and Yb in Ce(0.1)Lu(0.9)O(1.55-delta) and Ce(0.95)Yb(0.05)O(1.975-delta) samples heated at 1100 degrees C was studied by XANES and magnetic measurements. In the former Ce was dominated by Ce(4+), with small contribution of Ce(3+) after heating in H(2). In the latter, Yb existed exclusively as 3+ in both O(2) and H(2).