The gas-phase photochemistry of the diamines monomethylhydrazine (CH 3 NHNH 2 ) and unsymmetrical dimethylhydrazine ((CH 3 ) 2 NNH 2 ) has been studied in the ultraviolet (UV) at 298 K. Relative UV absorbance spectra of dilute mixtures of the diamine in He buffer gas were recorded in the region 140-310 nm with an instrument resolution of 0.25 nm. These were converted to absolute absorption cross section profiles by scaling the data with absolute 253.65-nm cross section values of σ 253.65 ) (8.022 ( 0.481) × 10 -20 and (35.883 ( 2.152) × 10 -20 cm 2 molecule -1 for CH 3 NHNH 2 and (CH 3 ) 2 NNH 2 , respectively, which we also determined in this work. Both compounds exhibit continuous diffuse absorption bands in the UV that correspond to electronic transitions to dissociative singlet excited states. Laser photodissociation product measurements revealed the absolute H atom primary quantum yields to be Φ 248 ) 1.07 ( 0.02, Φ 222 ) 0.99 ( 0.01, and Φ 193 ) 0.94 ( 0.07 for CH 3 NHNH 2 photolysis at 248, 222, and 193 nm, respectively. The corresponding values for (CH 3 ) 2 NNH 2 photolysis were Φ 248 ) 0.97 ( 0.06, Φ 222 ) 0.91 ( 0.06, and Φ 193 ) 0.90 ( 0.04. Kinetic measurements gave the absolute second-order H atom rate coefficients to be k ) (7.60 ( 1.14) × 10 -13 and (13.48 ( 2.02) × 10 -13 cm 3 molecule -1 s -1 respectively for reactions with CH 3 NHNH 2 and (CH 3 ) 2 NNH 2 at 298 K and in 26.0 Torr of He. Trends in the UV absorption cross section profiles, photodissociation product distributions, and the H atom reactivities in the homologous diamine series (CH 3 ) 2 NNH 2 , CH 3 NHNH 2 , and N 2 H 4 are discussed.