AbstractThe random coil dimensions of polysarcosine and poly‐α‐aminoisobutyric acid have been evaluated using both theoretical and experimental methods. Theoretical values for characteristic ratios were derived from the (ϕ, Ψ) potential energy maps for the cis and trans isomers of Nα‐acetylsarcosine N,N‐dimethylamide and for Nα‐acetyl‐α‐aminoisobutyryl N‐methylamide. Experimental values were derived from viscosity data in various solvents. For polysarcosine in glacial acetic acid, the experimentally derived characteristic ratio agreed with the theoretically derived value for the trans isomer. From the lower ratios observed using water and particularly chloroform, these solvents are presumed to admit the cis isomer. Theoretical calculations (derived from energy maps) of poly‐α‐aminoisobutyric acid gave an unusually low characteristic ratio (<1) which was not in agreement with that from viscosity measurements in heptafluorobutyric acid. Reasons for the discrepancy are discussed in terms of the ease of propagation of helical region