In a survey of UK consumers, we elicited their willingness to accept (WTA) a discount for GM foods and willingness to pay (WTP) a premium for non-GM foods in order to assess their valuation on the non-GM characteristic in food products.  Mean WTA is found to exceed mean WTP, suggesting the valuation of the non-GM characteristic reflects an endowment effect, imperfect substitutability between GM and non-GM foods, or both.  Regression results show that perceived risks (benefits) associated with GM foods significantly increase (decrease) WTA and WTP estimates.  Additional regression models using the difference between WTA and WTP as the dependent variable indicate that risk (benefit) perceptions increased (decreased) the discrepancy between WTA and WTP estimates.  The role of risk perceptions in explaining this discrepancy is congruent with consumers' propensity toward loss aversion as predicted by the endowment effect hypothesis and prospect theory.