A gas-phase NMR kinetic, technique has been used for the first time to obtain accurate measurements of rate constants of some bimolecular, second-order cycloaddition reactions. As a test of the potential use of this technique for the study of, second-order reactions, the rate constants and the activation parameters for the cyclodimerization reactions of chlorotrifluoroethylene (CTFE) and tetrafluoroethylene (TFE) were determined in the temperature range 240-340 degreesC, using a commercial high-temperature NMR probe. Obtaining excellent agreement of the results with published data, the technique was then applied to the reaction of 1,1-difluoroallene with 1,3-butadiene, the results' of which indicate that the use of gas-phase NMR for reaction kinetics is particularly valuable when a reagent is available only in small amounts and in cases where there are several competing processes occurring simultaneously. The major processes observed in this reaction are regioselective [2+2] and [2+4] cycloadditions, whose rates and activation parameters were determined [k(2)= 9.3 x 10(6) exp(-20.1 kcal mol(-1)/RT) L/mol(-1) s(-1) and k(3) = 1.2 x 10(6) exp(-18.4 kcal mol(-1)/RT) L/mol(-1) s(-1), respectively] in the temperature range 130-210 degreesC.