Well-shaped single crystals of RNiO3 perovskites (R= Nd, Ho, Y) have been grown under high oxygen pressure conditions, in a piston-cylinder press at 2 GPa or in a belt-type press at 4 GPa. The reaction took place in sealed platinum capsules, in the presence of KClO3 as oxidizing agent. It seems that the choice of hydroxides of the involved cations as precursors is crucial for the success of the crystal growth, via water vapor transport reactions. The perovskites have been characterized by X-ray diffraction, scanning microscopy and DSC measurements, suitable to identify the metal insulator transition. The transition temperatures of the as-grown crystals show a significantly lower hysteresis in the heating and cooling runs than those reported for powder materials, suggesting a lower density of defects.