Raman spectra of poly(dG) poly(dC) have been observed In aqueous solutions at various Ionic strengths, [NaCl] = 0.03 to 4 M, and at different temperatures, 10 to 60 °C. At 30 °C, and at [NaCl] = 0.03 M, it was found to have a B-form (with O 4 ,endo-anti guanoslne and C 2 ,endo-antl cytldlne), whereas, at [NaCl] = 4 M, an A form (with C 3 ,endo-anti guanoslne and C 3 ,endo-antl cytldlne). At 30°C and [NaCl] = 1 M, namely at an intermediate state, a fraction of this molecules was considered to have a “heteronomous A” form (with O 4 ,endo-anti guanoslne and C 3 , endo-anti cytidlne). At 60 °C and [NaCl] = 1 M, It assumes the B form, and at 10 °C and [NaCl] = 1 M, the A form. Cytoslne-5-methylation was found to cause a marked stabilization of the A form. Even at [NaCl] = 0.1 M (at 30 °C), a substantial portion of poly(dG) -poly(dm 5 C) was found to have a heteronomous form, in which the dG atrand is in the B form and the dC an A form; it never assumes a complete B form.