We present a structural analysis of (\mathrm{NaAlAs}{2} \mathrm{O}{7}, M_{\mathrm{r}}=311.81), monoclinic, (P 2_{1} / c, a=6.9114(7), b=8.1345(12), c=9.5446(10)) (\AA, \beta=107.51(1)^{\circ}, V=511.74 \AA^{3}, Z=4, d_{\mathrm{m}}=4.02, d_{\mathrm{cal}}=4.05) (\mathrm{g} / \mathrm{cm}^{3}, \mathrm{Mo} K \alpha, \lambda=0.7107 \AA, \mu=140.3 \mathrm{~cm}^{-1}, F(000)=146), (R=0.039, R_{\mathrm{w}}=0.038) for all the 1023 independent observed reflections. The structure of (\mathrm{NaAlAs}{2} \mathrm{O}{7}) is compared to that of the (M^{\mathrm{I}} M^{\mathrm{III}} \mathrm{P}{2} \mathrm{O}{7}) form II diphosphates. The two structures are composed of the same building units, (M^{\mathrm{III}} \mathrm{O}{6}) octahedra and (\mathrm{X}{2} \mathrm{O}{7}) ( (X=) As or (P) ) groups. The main difference is that the parts played by the (\mathrm{X}{2} \mathrm{O}{7}) groups and the (M^{111} \mathrm{O}{6}) octahedra, as also the stacking of the polyhedra, are exchanged in the two structures. The change is shown to be likely due to the length of the As-O-As bridge and makes it possible to account for the unexpected contraction of the (a) parameter in (\mathrm{NaAlAs}{2} \mathrm{O}{7}) in spite of its being the lengthening direction of the (\mathrm{X}-\mathrm{O}-\mathrm{X}) bridge. (1) 1994 Academic Press, loc.