Two samples of com. and pure H(3)PO(4) were mixed with com. Na(2)CO(3) and calcined at 200-550 degrees C for 10-60 min to study the effect of calcination temp. and time on the phase compn. of the Na(5)P(3)O(10) formed. The P(2)O(5) content in the products of calcination of com. H(3)PO(4) samples increased with increasing calcination temp. while the product of calcinations of pure H(3)PO(4) showed a max. of the P(2)O(5) content at 300-350 degrees C. The calcination time did not play any important role in the formation of final products.