Tricoordinate phosphorus NMR chemical shifts are computed (GIAO/6È311 ] G**//RMP2(fc)/6È 31 ] G*) and analyzed for molecules (X, F, OH, BeH, Li and H \ "simple Ðrst PX 2 Y Y \ EH n : N H 2 , CH 3 , BH 2 , row substituentsÏ), the "normalÏ d(31P) relationship with the substituent electronegativity sum, ; EN (; i/1 3 EN(E i ) ; downÐeld shift with increasing ; EN) is substantiated for experimentally important molecules by the ab initio results. The "inverseÏ d(31P)È; EN trend obtained for X \ F is related to negative hyperconjugation (L p(X) ] p* (PÈX@ ) orbital interaction). The molecules display "normalÏ ; ENÈshift relationships when the electro-P(OH) 2 Y negativity of Y is low and "inverseÏ when the electronegativity of Y is high. For some phosphanes with simple PX 2 Y substituents, the d(31P) values are well related to ; EN. The best correlation (cc : 0.955) between d(31P) and ; EN is found for the set of monosubstituted phosphanes, The d(31P)ÈEN(Y) slope (in ppm per EN "unitÏ), ranges PH 2 X. from 162 (X \ BeH), 141 (X \ H) and 98 and to [105 (X \ F). The d(31P) values of the (X \ CH 3 X \ BH 2 ) molecules are representative for the complete set of PXYZ with simple substituents. Since there is no "simple, PX 2 Y general relationshipÏ of d(31P) for phosphanes with less or much more electronegative substituents, EN(E) \ EN(P) > EN(E), consequently, it does not exist in the complete set.