The relative flux creep rate S(t)≡d lnM/d ln t of a single crystal of Ba1-xKxBiO3-δ(BKBO) was measured up to a few degrees Kelvin below the irreversibility line. It is nearly constant at low temperatures but rises sharply after a threshold temperature Tt. Above Tt, the creep changes from logarithmic to non-logarithmic decay. All these are very similar to those observed in Y1Ba2Cu3O7-δ (YBCO) samples and suggest a flux-lattice phase sub-boundary. The ratio between the boundary field Bt and the irreversibility field Bi falls between 0.15 and 0.25 for both BKBO and YBCO at various temperatures, implying a possible close relationship between the boundary and the irreversibility line.
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