Interplanetary shock waves propagating through the solar wind have been observed with hemispherical plate electrostatic analyzers aboard the twin Vela 3 satellites. From the consideration of mass flux continuity, the shock velocities have been calculated; the shock velocities determined from the data in this manner are consistent with the observed temperature changes across the shocks. These velocities are significantly less than themeantransit velocities of the shocks from the sun to the earth derived from the delay times between the initiating flares and shock arrival. This result is interpreted to mean that interplanetary shock waves are decelerated as they expand and propagate through the quiescent solar plasma.