The gas-phase photochemistry of the diamines monomethylhydrazine (CH3NHNH2) and unsymmetrical dimethylhydrazine ((CH3)(2)NNH2) 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 sigma(253.65) = (8.022 +/- 0.481) x 10(-20) and (35.883 +/- 2.152) x 10(-20) cm(2) molecule(-1) for CH3NHNH2 and (CH3)(2)NNH2, 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 Phi(248) = 1.07 +/- 0.02, Phi(222) = 0.99 +/- 0.01, and Phi(193) = 0.94 +/- 0.07 for CH3NHNH2 photolysis at 248, 222, and 193 nm, respectively. The corresponding values for (CH3)(2)NNH2 photolysis were Phi(248) = 0.97 +/- 0.06, Phi(222) = 0.91 +/- 0.06, and Phi(193) = 0.90 +/- 0.04. Kinetic measurements gave the absolute second-order H atom rate coefficients to be k = (7.60 +/- 1.14) x 10(-13) and (13.48 +/- 2.02) x 10(-13) cm(3) molecule(-1) s(-1) respectively for reactions with CH3NHNH2 and (CH3)(2)NNH2 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 (CH3)(2)NNH2, CH3NHNH2, and N2H4 are discussed.