The F-type supergiant HD 56126 (F5I) is an IRAS source with detached cold dust shell with characteristics similar to the dust shells around planetary nebulae. From an analysis of high resolution and high signal to noise ratio spectra metal and CNO abundances have been determined. It is found that in HD 56126 [C/H] =−0.01, [N/H]=+0.17, [O/H] = −0.02, [S/H]=+0.01 and [Fe/H]< −1.0. The C/N/O abundance ratios and CNO abundances relative to Fe of HD 56126 and related post AGB stars suggest that they have on their photospheres the material processed by triple alpha, CN and ON cycles.