Four phosphate rocks of various origin were mechanochem. activated after addn. of clinoptilolite previously treated with NH(4)Cl. The activation was carried out in mortar or ball mills and resulted in an increase in rock soly. in aq. soln. of NH(4) citrate. The P(2)O(5) availability increased by more than 10 times with increasing the clinoptilolite/rock ratio (up to 20) and grinding time (up to 120 min).