Membrane phase behavior and fluidization have been examined in heterogeneous membranes composed of dipalmitoylphosphatidylcholine (DPPC, a saturated lipid) and dioleoylphosphatidylcholine (DOPC, an unsaturated lipid) at n-butanol concentrations below and above the interdigitation threshold of DPPC. Our results show that the presence of DOPC did not influence the interdigitation concentration of n-butanol on DPPC (0.1-0.13 M) despite the fact that DOPC increased n-butanol partitioning into the membranes. When DPPC was the continuous phase, up to equimolar DPPC:DOPC, n-butanol partitioning into gel or interdigitated DPPC was only slightly affected by the presence of DOPC. In this case a "cooperative effect" of DOPC + n-butanol eliminated the DPPC pretransition phase and yielded an untilted gel-like phase. When DOPC was the continuous phase, more n-butanol was needed to cause DPPC interdigitation (0.2 M), which was attributed to n-butanol residing at the interface between DOPC and DPPC domains. To our knowledge, this is the first study to examine the effects of n-butanol partitioning on membranes composed of saturated and unsaturated lipids that exhibit coexisting phase states.