We show that liquid polyoxacyclobutane -[CH2-CH2-CH2-O](n)- when mixed with water at room temperature precipitates solid cocrystals of the polymer and water. Cocrystals can also be formed by simply exposing the liquid polymer to saturated humidity. This appears to be the only known example of nonreacting liquids combining to form a solid cocrystal, also known as a clatherate, at room temperature. At high temperatures, the same polymer water mixtures phase separate into two coexisting liquid phases. This combination of cocrystal formation and LCST-type liquid liquid equilibrium gives rise to an unusual, possibly unique, type of phase diagram.