Molecularly nonstoichiometric crystals obtained as a result of differential occupation of sites with oxygen atom/phosphorus lone pair of electrons or with sulfur/selenium in three sets of phosphorus compounds are described. These are formed by a combination of (a) [CH2(6-t-Bu-4-MeC6H2O)(2)]PNMe2 (11) and [CH2(6-t-Bu-4-MeC6H2O)(2)]P(O)NMe2 (13), (b) [CH2(6-t-Bu-4-MeC6H2O)(2)]P(S)NMe2 (14) and [CH2(6-t-Bu-4-MeC6H2O)(2)]P(Se)NMe2 (15), and (c) [(2,6-Me2C6H3O)(O)P-mu-N-t-Bu](2) (16) and (2,6-Me2C6H3O)(O)P(mu-N-t-Bu)(2)P(O-2,6-Me2C6H3) (17). In the case of c, three different types of crystals with varying stoichiometry of 16 and 17 (1:9, 1:1.5, and 1:0.43) are obtained. The results are substantiated by the combined use of P-31 NMR spectroscopy and X-ray crystallography. These observations suggest that we should be cautious with regards to the purity of samples when syntheses involving the oxidation of P(III) systems are reported. It is also emphasized that the apparent P-X distances in some of these crystals cannot actually be taken as true bond lengths.