Difluorocarbene reacts with 2-methoxy and 2-carbomethoxy-7,7-dimethylnorbornadiene to give homo-1,4 adducts only. The relative rates of addition are compared with that for 7,7-dimethylnorbornadiene and are found to be 2.63, 0.045 and 1.0 respectively. Consequently, an electrophilic process is operating. Difluoro, dichloro and dibromocarbenes react with 7,7-dimethylnorbornadiene exclusively on the endo face to give 1,2 and homo-1,4 adducts in ratios of ≈0.1, 0.5 and 0.7 respectively. The homo-cheletropic reaction, compared to the competing cyclopropanation, is increasingly sensitive to the bulk of the carbene partner.