The cocrystallization and copolymerization of two pairs of diacetylenes R–C≡C–C≡C–R, (1) 1 BU [R=–CH2OCONHC4H9] and 1 EU [R=–CH2OCONHC2H5], and (2) 2A [R=–(CH2)2OH] and 3A [R=(CH2)3OH] was studied by differential scanning calorimetry and reflectance spectroscopy. Several novel features were observed. 1 BU always crystallizes into a mixture of two phases, blue (λmax=585 nm) and red (λmax=515 nm). 1 EU and 1 BU cocrystallize to form solid solutions as evidenced by singlet endotherms or by different reflectance spectra. λmax, the lowest energy optical transitions of partially polymerized cocrystallized mixtures, increases as the concentration of 1 EU increases to about 90%. Mixtures of 2A and 3A form amorphous solid solutions. The amorphous solid solutions crystallize in the solid state before melting. DSC shows an exotherm followed by an endotherm during heating. Partially polymerized 3A changes from blue→red when touched by human hands. The color change is due to melting of unreacted monomer.