A pulsed mass spectrometer is described which was constructed for the measurement of rate constants of ion-molecule reactions at thermal energies. The mass spectrometer could be operated with ion source pressures up to 10 torr. The reactions studied were condensation and attachment reactions which can only be observed at high pressures. Reactions of millitorr traces of methane in 3.5 torr of krypton were first studied to check the pulsing technique. The values of the rate constants for reactions (1) and (2) (both 1.0x10[superscript -9] cc/molecule sec.) were found to be close to previously [scientific equations] determined values and thus the validity of the pulse technique could be established. The ionic condensation reactions (3) and (4) were studied in the xenon-sensitized ionization of ethylene to gain information on the radiolysis of ethylene. The values for the rate constants k[subscript n] [scientific equations] were found to agree with values which had been estimated from results of a previous technique in which the reaction times were not well known k[subscript ] = 2.1x10[superscript -9], k[subscript 4] = 4.35x10[superscript -12] and k[subscript 6] = 1.14x10[superscript -12] (cc/molecule sec.). The values k[subscript k] for reaction (4) were measured as [scientific equations]. These had not been previously measured. The rapid decrease of rate constant with increasing ionic size was explained in terms of the + ion structure. The effect of Xe[superscript +] on the stabilization of the condensation product C[subscript 4]H[subscript 8][superscript +] was observed. Ethylene pressures were varied from 1x10-13] to 0.5 torr and xenon pressures from 0.5 to 8 torr. The rate constants and equilibrium constant for the attachment reaction (5), which appears to be of aeronomic interest, were [scientific equation] measured over the pressure range of 1 to 8 torr at temperatures from 298 to 341°K. The reaction was found to have third order kinetics. The rate constant for the forward reaction was measured as 2.90x10[superscript -30] cc[superscript 2] /molecule[superscript 2] sec. at 298°K with a negative activation energy of 2.0 kcal/mole. The reverse rate constant was 2,44x10[superscript -13] cc/molecule sec. at 298°K. The values of the equilibrium constant 0.385 (standard state 1 torr), entropy -22.4 cal/mole deg. and enthalpy -9.98 kcal/mol. agree vejry well with previous results obtained under equilibrium conditions. Attachment reactions in nitrogen v/ere studied at pressures from 0.4 to 3.6 torr at temperatures from 297 to 420°K. A forward rate constant for the attachment (6) was measured as 8.3x10[superscript -29] cc[superscript 2] /molecule[superscript 2] sec. at 298°K with a negative activation energy [scientific equation] of 2.3 kcals/mole. These values support previous observations. The equilibrium constant could not be measured. Under identical conditions as those used above/ the attachment reaction (7) displayed second order kinetics. The [scientific equation] rate constant was measured as 1.5x10[superscript -12] cc/molecule second. No temperature dependence was observed. The rate constants and equilibrium constants for the oxygen and nitrogen reactions were compared by means of theoretical equations.