Solutions of hydrazine hydrate are vaporized, without any concomitant decomposition thereof, by heating such solutions, in the liquid phase, under conditions as to effect the decompression or volatilization thereof; this process is particularly applicable for the vaporization of that hydrazine hydrate solution contained in the reboiler of a column wherein the hydrolysis of an azine R.sub.1 R.sub.2 C.dbd.N-N.dbd.CR.sub.1 R.sub.2 is carried out, with recovery of hydrazine hydrate at the base of the column and of a ketone, R.sub.1 R.sub.2 C.dbd.O, at the column head.