3,8-Dimethyl-2,7-nonadienoic acid has been prepared. Its structure corresponds to that of geranic acid with the only difference that it contains an additional CH2-group in the middle of the chain. By the action of a mixture of formic and sulfuric acids a cyclic isomer, analogous to α-cyclogeranic acid but containing a seven-membered ring, is obtained in low yield. The main reaction product is a saturated