Deuterium α-secondary kinetic isotope effects for the ring opening of ethylene oxide with seven different nucleophiles are calculated at RHF/6-31G*, RHF/6-31+G*, MP2/6-31G*, MP2/6-31+G* and MP2/6-311+G(2d) levels of theory. The theoretical isotope effects are compared with optimized transition state geometries and the absolute value of the isotope effect is found to decrease as the transition state becomes more product-like, which for this system correlates with getting tighter. It appears that the isotope effect mainly is a measure of the loose/tight nature of the transition state, and less of the early/late feature. Within a limited class of nucleophiles there is an almost linear correlation between transition state geometry and isotope effect. This correlation is mainly due to bending vibrations, while the absolute value of the isotope effects is determined by stretching vibrations. Tunnelling calculations on two reactions show only minor corrections.