A polyether- or polyester-epoxide polymer (PEEP) composition which comprises a reaction product of: (a) a polyepoxide compound having an equivalent weight within the range of 115 to 250 g/eq.; and (b) a polyol composition comprising a polyester polyol or a mixture of a polyester polyol and a polyether polyol, wherein the polyester polyol has an acid number of less than 5 mg KOH/g, and wherein the polyol composition has a melting point within the range of 20°C to 100°C and a hydroxyl number from 10 to 30 mg KOH/g; wherein the ratio of epoxy equivalents of the polyepoxide compound to hydroxyl equivalents of the polyol composition is within the range of 2:1 to 6:1; wherein the amount of the polyepoxide compound is within the range of 5 to 30 wt.%, based on the amount of PEEP composition; wherein the amount of the polyol composition is within the range of 70 to 95 wt.%, based on the amount of PEEP composition; and wherein the PEEP composition is a solid-solid phase-change material having, as measured by differential scanning calorimetry at a heating/cooling rate of 10°C/minute, a transition temperature within the range of -10°C to 70°C, the transition temperature being a temperature or temperature range within which the PEEP composition undergoes a morphological change from one solid form to a different solid form, a latent heat at the transition temperature within the range of 30 to 200 J/g, and little or no detectable hysteresis or supercooling upon thermal cycling over at least five heating/cooling cycles that encompass the transition temperature. (a) a polyepoxide compound having an equivalent weight within the range of 115 to 250 g/eq.; and (b) a polyol composition comprising a polyester polyol or a mixture of a polyester polyol and a polyether polyol, wherein the polyester polyol has an acid number of less than 5 mg KOH/g, and wherein the polyol composition has a melting point within the range of 20°C to 100°C and a hydroxyl number from 10 to 30 mg KOH/g; wherein the ratio of epoxy equivalents of the polyepoxide compound to hydroxyl equivalents of the polyol composition is within the range of 2:1 to 6:1; wherein the amount of the polyepoxide compound is within the range of 5 to 30 wt.%, based on the amount of PEEP composition; wherein the amount of the polyol composition is within the range of 70 to 95 wt.%, based on the amount of PEEP composition; and wherein the PEEP composition is a solid-solid phase-change material having, as measured by differential scanning calorimetry at a heating/cooling rate of 10°C/minute, a transition temperature within the range of -10°C to 70°C, the transition temperature being a temperature or temperature range within which the PEEP composition undergoes a morphological change from one solid form to a different solid form, a latent heat at the transition temperature within the range of 30 to 200 J/g, and little or no detectable hysteresis or supercooling upon thermal cycling over at least five heating/cooling cycles that encompass the transition temperature. wherein the ratio of epoxy equivalents of the polyepoxide compound to hydroxyl equivalents of the polyol composition is within the range of 2:1 to 6:1; wherein the amount of the polyepoxide compound is within the range of 5 to 30 wt.%, based on the amount of PEEP composition; wherein the amount of the polyol composition is within the range of 70 to 95 wt.%, based on the amount of PEEP composition; and wherein the PEEP composition is a solid-solid phase-change material having, as measured by differential scanning calorimetry at a heating/cooling rate of 10°C/minute, a transition temperature within the range of -10°C to 70°C, the transition temperature being a temperature or temperature range within which the PEEP composition undergoes a morphological change from one solid form to a different solid form, a latent heat at the transition temperature within the range of 30 to 200 J/g, and little or no detectable hysteresis or supercooling upon thermal cycling over at least five heating/cooling cycles that encompass the transition temperature.