Using CD we investigated the transitions of poly (dCdG). poly(dGdC1 from B-to-2 form and from 2-to-2' form. We have found experimental conditions that allow the cooperative transition to occur as a function of temperature in ethanolic solutions. The transition is reversible and can be repeated as often as desired. There is no evidence of strand separation during the cooperative transition as monitored by absorbance. For purposes of calculation, we have assumed a two-state model for the B-to-2 transition, although the data indicate that such a model is too simplistic. The calculations allow the estimation of the change in enthalpy per mole of cooperative unit for the transition as a function of ethanol concentration. The values range from F140 to *ZOO kcaI/mol for ethanol concentrations between 10 and 20%. Investigations of the noncooperative Z-to-2' transition show that it is a reversible two-state transition. The different forms of poly(dCdG). poly(dGdC) give no scattering contributions to the CD as shown by fluorescent-detected CD or fluorscat techniques. This indicates that the CD spectra are true spectra, and contain no contributions from differential scattering of the polynucleotide. This is particularly significant in the case of the 2' form, since it exists a t high ethanoI concentrations (80%) where condensation of polynucleotides can provide large contributions to the CD spectra. Analogous investigations using methanol show that the two transitions also occur, but the final Z' form in methanol is qualitatively different from the ethanol form.