The substitution of one nitrogen for oxygen in the K2NiF4 structure type Oxides A2BO4 leads to the two isostructural A2(III)B(III)O3N and A2(II)B(IV)O3N series, illustrated by Nd2AlO3N and Sr2TaO3N which can be compared with the corresponding oxides CaNdAlO4 and Sr2TiO4. A comparative study of the c/a ratios and of the thermal expansion anisotropy allows to determine the anionic ordering. Nitrogen and oxygen are ordered in Nd2AlO3N, whereas, in Sr2TaO3N, a statistical occupancy of die same crystallographic site is observed.