AbstractWe have tested 21 different basis sets of synthetic DNA circular dichroism spectra and have slected one for use in spectral analyses of natural DNAs. This “standard” set consists of spectra of eight polymers: poly[d(A‐A‐T)·d(A‐T‐T)], poly[d(A‐G‐G)·d(C‐C‐T)], poly[d(A‐T)·d(A‐T)], poly[d(G‐C)·d(G‐C)], poly[d(A‐G)·d(C‐T)], poly[d(A‐C)·d(G‐T)], poly[d(A‐T‐C)·d(G‐A‐T)], and poly[d(A‐G‐C)·d(G‐C‐T)]. This basis set, applied according to the first‐neighbor polymer procedure of Gray and Tinoco, allows a more uniformly accurate spectral analysis of six natural complex DNAs and eight (A+T)‐rich satellite DNAs for base composition and first‐neighbor frequencies than was previously possible. We find that spectra of poly[d(A)·d(T)] and/or poly[d(A‐C‐T‐)·d(A‐G‐T)]are not generally required for good analysis results but we show in this and the following paper that t