By altering solvent/nonsolvent ratio and temperature, a comprehensive study is carried out to understand the molecular aggregation of poly[(1,3-phenyleneethynylene)-alt-oligo(2,5-dialkoxy1,4-phenyleneethynylene)]s 1 and 2 in solution, which possess different lengths of linear phenyleneethynylene (PE) fragments. Although both polymers exhibit random-coil conformation in THF solution, PPE 2 of longer rigid-rod fragment exhibits a higher tendency to form aggregate than 1. In a good solvent, optical spectra of PPEs reveal a gradual spectral shift in responding to the temperature change, which is attributed to molecular conformational change (planarity of chromophore). With addition of methanol to polymer solutions in THF, UV-vis absorption spectroscopy detects the aggregation band from 2 but not from 1. The combination of solvent/nonsolvent and low temperature provides improved sensitivity to probe molecular aggregation. Although chromophore planarity is anticipated to play an important role, current results favor the pi-pi interaction in the aggregate formation of 1 and 2.