YMnO3 attracted significant attention because of its multiferroic properties and the "geometric origin" of its ferroelectricity. Despite numerous studies of this compound, the structural changes that underlie the isosymmetric phase transition, which has been identified with the Curie temperature, remain unclear. Our Rietveld refinements using the variable-temperature neutron diffraction data indicate significant worsening of the fit below the isosymmetric transition that cannot be explained using the average-structure model. The aim of this proposal is to (1) use total scattering to determine the differences between the local and average structures of YMnO3 to clarify the nature of the isosymmetric phase transition and (2) determine the structural changes that underlie an anomalous atmosphere-dependent behavior of the lattice parameters with temperature in the Ti-substituted YMnO3.