Structures have been determined for a homogeneous series of five phosphate triesters (2a)-(2e). MoKa, 2=0.71093A, 293K. (2a): M r= 214.16, orthorhombic, Pna2j, a=15.453(3), b= 11.005 (2), c --5.925 (2) A, V-1007.6 (7) A a, z = 4, D x = 1.41 Mg m -a, # = 0.25 mm -1, F(000) = 448, R =0.045 for 2146 unique observed reflections; (2b): M r = 259.16, orthorhombic, P2~2121, a = 5.710 (2), b = 9.042 (2), c = 21.409 (4) A, V= 1105.3 (6) A a, Z = 4, D x = 1.56 Mg m -a, # = 0.26 mm -1, F(000) = 536, R=0.037 for 2434 reflections; (2c): Mr= 259.16, triclinic, Pi, a = 7.394 (2), b = 7.970 (2), e=9.966 (3)A, a=86.68 (2), fl=70.73 (2), y= 85.06 (2) °, V = 552.1 (6) A 3, Z = 2, Ox= 1.56Mgm -3, #=0.26mm -1, F(000)=268, R= 0.047 for 2250 reflections; (2d): M r = 293.61, triclinic, ei, a = 6.226 (2), b = 9.586 (2), c = 10.148 (2) A, a=93.77 (2), fl=92.51 (2), y= 100.96 (2) ° , v= 592.3 (6) A 3, z = 2, D x = 1.65 Mg m -~, /t = 0.47 mm -~, F(000)= 300, R = 0.046 for 2110 reflections; (2e): Mr= 304.16, triclinic, Pi, a = 5.547 (2), b = 10.382 (2), c = 10.696 (2) A, a = 87.01 (2), fl = 89.63 (2), y = 79.90 (2) °, V= 605.6 (6) A 3, Z = 2, D x = 1.67 Mg m -3, /t = 0.26 mm -~, F(000) = 312, R = 0.044 for 2425 reflections. All five compounds are in the expected chair conformation with the aryloxy group axial and significant flattening at phosphorus. The P-OAr bond length increases to a limiting value of 1.600 A as the aryl group becomes more electron withdrawing. (2a)-(2d) show a linear relationship between P-OAr bond length and reactivity; (2e) shows a negative deviation from this linearity. Further trends are (i) a shortening of the O--Ar bond and (ii) a change of the torsion angle about O-Ar from synclinal towards coplanar.