The phenomenon of a disorder-order transition upon cooling a solution or spin-coating a film, and the subsequent formation of aggregates, is often regarded as specific to P3HT. It is well known that it plays a major role in controlling the efficiency of bulk heterojunction solar cells of P3HT with fullerene. However, this disorder-order transition can be found in a range of chemically different polymers, and it proceeds in an analogous manner for P3HT, MEH-PPV, PCPDTBT, PFO and for the low-molecular weight compound p-DTS(FBTTh2)2. Employing absorption and fluorescence spectroscopy, we find that upon cooling a solution, the chain first swells up, then undergoes a transformation to aggregate and finally increases the conjugation length of the aggregate. Existing differences relate to the temperature at which the disorder-order transition proceeds, the sharpness of the temperature-dependent transition, the maximum fraction of aggregation obtained and the predominance of H- or J-type character in the spectra. For P3HT, we observe the formation of two different polymorphs that we attribute to two H-type aggregates with planar polymer backbones yet different degree of order regarding their side chains.