To provide a composition for a heat cycle system having less influence over the ozone layer, having a low global warming potential and having excellent durability, and a heat cycle system.A composition for a heat cycle system comprising a working fluid containing trifluoroethylene and difluoromethane, and a lubricating oil, wherein the interaction distance (Ra1123) between trifluoroethylene and the lubricating oil as determined from the Hansen solubility parameters is shorter than the interaction distance (Ra32) between difluoromethane and the lubricating oil.