Wide-line n.m.r. has been used to obtain the order parameter, S, as a function of temperature, for nematic p-methoxybenzylidine-p-n-butylaniline (MBBA) containing solutes of differing size and shape. These consisted of : (i) anisotropic molecules, capable of correlating their orientations with MBBA molecules, viz n-hexane, n-hexadecane and trans-decalin, and (ii) isomers of more isotropic shape, apparently incapable of orientational correlation, viz 2,2-dimethylbutane, 2,2,4,4,6,8,8-heptamethylnonane, and cis-decalin. With the same mole fraction, x₂, of anisotropic solute, there is the same lowering of S while the lowering with the isotropic solutes increases with solute size. The parameter S is > 0.36 in the one-phase nematic region in the phase diagram and is equal to 0.36 along both the (T, x₂)^N line where the isotropic phase first appears on heating and the (T, x₂)^I line where the nematic phase appears on cooling, i.e., S= 0.36 through the two-phase region. For all systems, S is a single function of the ratio T/T^N. Phase diagrams have been obtained for 21 MBBA + solute systems. Solutes include the anisotropic, orientation-correlating normal alkanes, n= 6, 8, 12, 16, 20, 24 and trans-decalin and a variety of more isotropic, non-correlating solutes : the highly branched alkane isomers of C_n, n= 6, 8, 12 and 16; cycloalkanes; cis-decalin; SnX₄, where X = methyl, butyl, octyl and lauryl; Pb(butyl)₄ and dimethylsiloxane oligomers. Identical (T, x₂) phase diagrams are obtained for all n-alkane solutes, their effect on the nematic-isotropic transition being independent of their molecular dimensions. This implies that the molecular cross-sectional area is the relevant factor for these anisotropic solutes. For the isotropic solutes, in contrast, there is a strong dependence on solute size of the (T, x₂)^N and (T, x₂)^I lines. Values of the slopes of T^N and T^I against solute mole fraction are corrected to infinite solute dilution. The results are compared with predictions of lattice model, virial expansion and other theories. They are also used to give the difference of solute activity coefficients in the nematic and isotropic MBBA(γ^N∞₂/γ^I∞₂– 1). This quantity is independent of size for the normal alkane solutes but increases rapidly for the branched alkanes and other isotropic solutes.