A stability study of perovskitic LaREO3 oxides (RE = Dy, Ho, Er, Tm, Yb, Lu) as a function of temperature was undertaken. A correlation between the Goldschmidt t value and the perovskitic stability field amplitude was found: the latter widens as t increases. Magnetic measurements, performed on all the perovskitic samples, showed that t is also related to the exchange interactions. LaREO3 oxides were synthesized by thermal decomposition of the corresponding coprecipitated oxalates at temperatures ranging between 600 and 1800 °C. Simultaneous differential thermal and thermogravimetric analyses showed that all the La–RE mixed oxalates decompose similarly. All the oxides, except LaDyO3, crystallize in the perovskitic form in a temperature range that depends on the ionic size difference between La and the smaller rare earth; above and below the perovskitic stability field, the B or C form, typical of rare earth sesquioxides, is present. Rietveld refinements, carried out on all the LaREO3 samples synthesized at 1200 °C, showed the occurrence of an orthorhombic distorted perovskitic structure belonging to the Pnma space group.