Spin-polarized metastable atoms of ultracold ytterbium are trapped at high density and their inelastic collisional properties are measured. We reveal that in collisions of Yb( 3 P2) with Yb( 1 S0) there is relatively weak inelastic loss, but with a significant spin dependence consistent with Zeeman sublevel changes as being the dominant decay process. This is in strong contrast to our observations of Yb( 3 P2)–Yb( 3 P2) collisional losses, which are, at low field, much more rapid and have essentially no spin dependence. Our results give a guideline to using the 3 P2