  We examined the effect of Pb substitution in the layered superconductor
LaO0.5F0.5Bi1-xPbxS2 (x=0~0.15) through the measurements of the resistivity,
thermal expansion, specific heat, and Seebeck coefficient. These transport and
thermal properties show anomalies at certain temperatures (T*) for
x${\geq}$0.08. The large thermal expansion anomalies, specific heat anomalies,
and the existence of hystereses in the above measurements indicate a
first-order structural phase transition at T*. Additionally, the Seebeck
coefficient indicates that the anomalies at T* are related not only to the
lattice system, but also to the electronic system. Superconductivity is not
observed above 2 K at x=0.08, which is around the phase boundary where T*
vanishes. The suppression of superconductivity around the structural phase
boundary suggests a close relationship between the lattice and
superconductivity.
