In order to study the spatial and temporal evolution of the post-flare coronal loops observed during two ribbon flares, we have analyzed in detail two events: May 21 1980, 20:55 U.T. and Nov. 6 1980, 14:50 U.T. We have used the set of X-ray (2.5-30 keV) data provided by the Hard X-ray Imaging Spectrometer (HXIS) aboard the Solar Maximum Mission (SMM) Satellite. The isotherms for different times have been obtained, from the count rates in the low energy bands, using the count rate prediction program (CRPP). In both events we have observed that the hottest regions lay at the greatest heights, which is compatible with the theoretical models that predict that the reconnection of opened magnetic fields lines proceeds from low to high altitudes in the solar corona.