First principles computational studies have been carried out on the gas phase and surface polymerisation of formaldehyde. Any gas phase polymerisation in dark conditions is shown to occur by way of neutral species, as opposed to earlier suggestions that a charged species may be involved. The resulting paraformaldehyde chains are shown to be stable only when fully bonded at both chain ends, as opposed to some of the earlier suggested mechanisms. Polymerisation is shown to be thermodynamically favourable in the gas phase, but kinetically limited. Derived rate constants are insufficient to account for experimental results, in agreement with prior studies suggesting that gas phase polymerisation actually occurs on surfaces rather than directly in the gas phase. The TiO₂ (110) surface is employed as a model surface to study formaldehyde adsorption and possible polymerisation mechanisms. Formaldehyde monomers are shown to weakly bind by way of coordinate bonding through the carbonyl group to surface Ti. A particularly strong dimerisation configuration is also found where the two ends of paraformaldehyde are terminated by a surface Ti and surface O, consistent with the earlier observation that paraformaldehyde chains are only stable when saturated at both ends.