The orientation relations m(100) parallel to t(001), m[001] parallel to t[110]; m(011) parallel to t(100), m[100] parallel to t[001]; m(100) parallel to t(110), m[001] parallel to t[001]; m(013) parallel to t(116), m[001] parallel to t[001] (indices for the primitive tetragonal cell) have been found between the tetragonal (t) and monoclinic (m) domains during the electron irradiation-induced m-t phase transition observed in-situ with HREM within isolated zirconia nanoparticles. Geometric models of the m-t interfaces are proposed. (C) 2004 Kluwer Academic Publishers.