89Y NMR has been used to study the effect of a range of substitutions (V, Fe, Co, Ni, Zn, Al and Ga) for Cu in YBa2(Cu1-xMx)3O7. A different behaviour is observed for the substitutions (Fe, Co, Al and Ga), which go into the site and change the structure to tetragonal, compared to the other substitutions (Ni and Zn). For chain site substitutions the room temperature shift increases linearly with concentration and the relaxation rate decreases whilst the shifts become more positive with decreasing temperature. For these substitutions the shift and relaxation appear to obey a Korringa relation and the reduction in Tc is correlated with the change in shift indicating that the suppression of Tc is due to the changes in N(EF). Plane site substitutions have only a small effect on the shift, whilst increasing the relaxation rate at room temperature. The increased relaxation rate indicates that some form of magnetic scattering is produced by these substitutions and this combined with the small changes in shift indicates that magnetic effects rather than hole doping are the cause of the reduction in Tc. In addition, MAS NMR data for 51V and 27Al in V and Al substituted YBa2Cu3O7 are presented. Both the 89Y behaviour and the 51V data suggests that vanadium substitutes on the chain sites. © 1992.