Two series of NZP compounds, NaM2(PO4)3 and NbM(PO4)3 [M = Ti, Zr, Hf, Sn and Th] have been synthesised and their thermal expansion coefficients in the temperature range 298 - 1273 K determined by using high temperature x-ray powder diffractometry. In the first series, the influence of the M - O bond is clearly seen even in the presence of the large contribution of the Na - O bond to thermal expansion; the expansivity of the lattice is found to vary with the ionic radius of the M atom. This trend is also reflected in the infra-red absorption frequencies of these compounds. In the second series, while the expansion behaviour is found to be dependent on M, the variation of expansivity with M is different from that found in the first series. The tin compounds in both the series display anomalous behaviour. We also report here, for the first time, a phase transformation in NbSn(PO4)3 around 673 K characterized by a maximum and a minimum in the lattice parameters a and c respectively.