Acid aluminum phosphates have been obtained by precipitation in aqueous solution. The influence of two important synthesis parameters, pH and ageing time, has been studied and important differences in the final materials have been observed. All precipitated phosphates present acid character regardless of pH and ageing time. An increase of phosphate acidity, crystallinity and size is favored with decreasing pH. Moreover, at synthesis pH below 4.8, increase of these characteristics have been also observed with increasing ageing time. In addition, the growth mechanism of phosphate particles has been established as a mechanism based on aggregation of globular primary nanoparticles regardless of the size and morphology of the final powders.