The electron spin resonance spectrum of a quasi 1D S=1/2 antiferromagnet K2CuSO4Br2 was found to demonstrate an energy gap and a doublet of resonance lines in a wide temperature range between the Curie--Weiss and Ne`{e}l temperatures. This type of magnetic resonance absorption corresponds well to the two-spinon continuum of excitations in S=1/2 antiferromagnetic spin chain with a uniform Dzyaloshinskii--Moriya interaction between the magnetic ions. A resonance mode of paramagnetic defects demonstrating strongly anisotropic behavior due to interaction with spinon excitations in the main matrix is also observed.
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