A Feulgen-densitometric comparison of nuclear DNA contents (C-values) was performed in various plant species (a fern, four gymnosperms, 16 woody and herbaceous angiosperms) after two types of fixation, additive (neutral formaldehyde) and non-additive (methanol-acetic acid, 3:1, MAA). Nuclei from tissues containing a significant amount of polyphenols (of the hydrolysable and non-hydrolysable tannin type) always showed reduced stainability and distorted spectral absorbance curves after MAA-fixation, while after formaldehyde-fixation no evidence for distorted staining was found. No fixation-dependent differences in Feulgen-DNA contents were stated in nuclei from tissues having no polyphenols. Distorted Feulgen-staining is a consequence of cellular self-tanning during fixation. Tanning is impaired by formaldehyde which binds to tannins and inactivates them. The rationale for using formaldehyde as a fixative in Feulgen-cytophotometry can be mainly seen in its capability of eliminating the “self-tanning error”. Standardization in plant DNA cytophotometry, and recent reports on unorthodox nuclear DNA variation in conifers are critically discussed.