Proton and ³¹P NMR. spectra in solution of several TiCl₄-PR₃ systems have been measured at variable temperatures. The results suggest the presence of fast equilibria in solution involving 1:1 and 1:2 adducts, and free tertiary phosphine. In addition there is evidence for the probable existence, at low temperature, of binuclear species of the type Ti₂Cl₈(PR₃). Correlations are found between the ³¹P chemical shifts for the 1:1 adducts on the one hand, and Taft 's constants and half‐neutralisation potentials on the other. Some new 1:1 and 1:2 TiCl₄-PR₃ adducts have been isolated and have been characterised by analysis and infrared spectroscopy.