The crystal structures of perovskites NdCoO3 and PrCoO3 have been studied by neutron powder diffraction over a temperature range from 100 to 1000 K. An anomalous isotropic expansion of CoO6 octahedra is observed at two successive temperatures corresponding to the spin transitions of Co3+ from low spin (LS) (S=0) to high spin (HS) (S=2) and intermediate spin (IS) (S=1) . At the second spin transition, a significant change in the octahedral tilt is encountered, as well. The heat capacity measurements show broad excess peaks at both transitions and the integral entropy change for each of them is estimated to be ≈10JK-1mol-1 . The structural and specific heat anomalies are interpreted within the LS-LS/HS-IS model of the Co3+ spin transitions.