Herein we report on the preparation and characterization and on some aspects of the chemistry of [Os(NH3)4-(eta2,eta1-(CH2=CHCO2H)2]2+ (1) and [Os(NH3)4(eta2-CH2=CHCO2H)(H2O)]2+ (2) in aqueous solution. The pK(a) values, as determined by titration, for 1 are 3.20, 6.78, and 9.8 and for the corresponding Os(III) form are 1.76, 3.46, and 7.40. In each case they refer to deprotonation of eta2-acrylic acid, eta2-acrylic acid, and an ammonia, respectively. For 2, the values are 2.75, 5.27, and 8.57, and for the corresponding Os(III) form the values are 1.42, 3.65, and almost-equal-to 8. Here they refer to deprotonation of the coordinated H2O, Of the eta2-acrylic acid, and of an ammonia, respectively. Particularly for Os(II) they illustrate the powerful electron-withdrawing effect of the alkene link. The 3+/2+ redox potentials of 1 and 2 in acid are 0.59 and 0.69 V vs (SHE), respectively, and they change with pH in accord with the value of pK(a). The aquation of 1 to form 2 is accelerated by HPF6, but the effect reaches a limit. This behavior together with other observations lead us to the conclusion that opening of the eta2-bond precedes protonation and that protonation then takes place at the pid electrons, thereby labilizing for substitution. With HCl and HO3SCF3, complications occur which we ascribe to addition of the anions to the beta carbon after the eta2-linkage opens. In these cases protonation of the pid electrons leads to the formation of monohydrides of Os(IV)(paramagnetic), without loss of the eta1-ligand.