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We present Very Long Baseline Array observations of the kinematics of thewater masers associated with OH 12.8-0.9, the fourth member of the so-called"water fountain" class of sources. We find that the masers occupy two distinctregions at the ends of a bipolar jet-like structure oriented north-south, withthe blue-shifted masers located to the north and the red-shifted masers to thesouth. The masers are distributed along arc-like structures 12-20 mas acrossoriented perpendicular to the separation axis with an angular separation of~110 mas on the sky. Our multi-epoch observations, show the two maser arcs tobe expanding away from each other along the axis of separation. The relativeproper motions of the two maser regions is 2.7 mas/yr (~105 km/s at the assumeddistance of 8 kpc). The measured radial velocity difference between thenorthern, blue-shifted masers and the southern, red-shifted masers is 48.4km/s. The radial velocity, when combined with the proper motion, yields athree-dimensional expansion velocity of 58 km/s and an inclination angle of 24deg. for the jet. By combining our radial velocities with historical values, weestimate the three dimensional acceleration of the masers to be ~0.63 km/s/yrand a dynamical age for the collimated outflow of ~90 yr.

Water Maser Kinematics in the Jet of OH 12.8-0.9