A dispertion relation for the process ω→π+γ is obtained by the analytical continuation in the ω mass. Some problems concerning dispertion relations involving unstable particles arc discussed. Under simplified assumptions we obtain Wω→π+r/Wω→3π≈0.35 and Wη→π+r/Wη→3π≈25, if η has the same quantum numbers as ω. The effect of the interaction in pairs between three pions is included in the calculation.