In this paper we present our recent positron annihilation study of the liquid»solid phase boundary for CO2 confined in nanometer pores of VYCOR glass. We find that CO2 remains liquid in the pores far below the bulk freezing temperature and there is pronounced hysteresis between freezing and melting compared to that seen at the gas-liquid boundary in the pores. On freezing we see evidence of open space created in the pores. This leads to complex melting behaviour possibly involving the formation of gas-liquid interfaces. We see that frezing in the pores is totally irreversible, so that any solid which forms (no matter how small) remains stable up to the higher melting temperature. In contrast melting is more reversible (possibly indicating nucleation centres which permit immediate re-freezing). Finally, the pre-frozen state in the pores is different to the post-melted state.