AbstractPhosphate frequencies for eight nucleoside 3′,5′‐ and four nucleoside 2′, 3′‐cyclic monophosphates have been determined by Raman and infrared spectroscopy based on comparisons with modified cyclic nucleotides and model cyclic phosphates. To a first approximation the symmetric diester stretching frequency, νs(OPO), is determined by the OPO bond angle. However, substitution on the phosphate ring also affects the frequency and intensity of νs(OPO). These results are discussed in terms of the use of νs(OPO) to determine the degree of backbone order in p