We have performed a systematic 151 Eu-Mössbauer and powder X-ray diffraction (XRD) study on two types of zirconia-based defect-fluorite-type solid solutions (a) Eu 2 (Ce 1−x Zr x ) 2 O 7 (0≤x≤1.0) and (b) LnEuZr 2 O 7 (Ln=La, Nd, Sm, Gd, Dy, Ho, Y, Er, Tm) at ideal-pyrochlore composition (A 3+ 2 B 4+ 2 O 7 ). The XRD results show that with increasing ratio of average A, B cations' radii (r A /r B ), the crystal structure of both of these two systems changes from the disordered defect-fluorite (DF) type to the ordered pyrochlore (P) type at around r A /r B =1.435 (x=0.85) for (a) and 1.46 (between Gd and Dy) for (b). The Eu 3+ Mössbauer parameters of these two systems, isomer shift (IS) and quadrupole splitting (QS), have been found to exhibit a clear tendency that with increasing r A /r B both Mössbauer parameters decrease first gradually in the DF-phase region and then much more steeply in the P-phase region. The more steep change of both the IS and QS in the P-phase region coincides with the XRD-obtained crystallographic data that both the average Eu–O bond length and the distortion of oxygen coordinated polyhedron around Eu 3+ increase with increase in r A /r B .