The atmospheric chemical behaviour of 1,1,1,2-tetrafluoroethane (CF,CFH,, HFC-134a) with respect to its rate and mechanism of degradation in the troposphere has been investigated. The rate coefficient for the reaction of (1 a) CF3CFH2 + OH+CF3CFH + H,O has been determined in direct time-resolved experiments using laser-pulse initiation and laser long-path absorption. A value of k l a = (4.6+0.5)x lo-'' cm3/s at T = 295 K has been found. The ratio of the rate coefficients for the reactions of the CF3CFHO-radical with O,, (4) CF3CFHO+02-tCF3CFO+H02, and C-C bond fission, ( 5 ) CF,CFHO+M-+CFHO+CF, +M, for T = 295 K and ptotai = 50 mbar (0,) has been obtained to be k4/(k, [MI) = 1.5 x lo-'' cm3, with the individual values being k, = 2 . 7~ lo-'' cm3/s and k, [MI = 1 . 8~ lo4 s-'. The mechanism of the atmospheric degradation of HFC-134a has been investigated in the temperature range 244-295 K and ptotal = lo00 mbar using UV photolysis/FTIR product studies. From these experiments the Arrhenius expression k,/(k, [MI) = 8.7x exp ((32401% K/T) cm3 was determined. Moreover, it is concluded that the major carbonyl products are CF3CF0, CFHO and CF20 which result from the two possible reactions of the oxy radical CF,CFHO. As a consequence the yield of these carbonyl products is dependent on temperature, 0, partial pressure and total pressure. For the reaction of CFHO with C1 atoms the Arrhenius expression k18 = 5 . 7~ 10-14exp (-(1130 k 160) K / T ) cm3/s was determined in the temperature range 244-298 K.