Disorder diffuse scattering has been observed for N-oxyphenazine which indicates the existence of domains of short-range order in (100) planes, of average size 13 angstrom (along b) by 50 angstrom (along c). Along a the alternation of centrosymmetrically arranged molecules is random. Mixed crystals with phenazine show diffuse scattering which indicates that up to about 60 mole% of phenazine can occur in the structure without disrupting these short-range-order domains. Mixed crystals of low N-oxyphenazine content have these molecules arranged randomly in the phenazine matrix. Between these two phases there appears to be a miscibility gap. It is probable that two components, one of which is ordered and the other showing only short-range order cannot form a continuous mixed-crystal series.