The interaction of detergents with fluorescent dyes was applied for the quantitative determination of SDS, used in the manufacture of biological products. Positively charged dyes, like ethidium bromide (EB) were applied at neutral pH, while in acid pH neutral dyes, that acquired their charge by protonation, like tiochrome, have been applied. Limits of detection are reported. The changes were monitored by the change of a suitable fluorescent parameter resulting from the nature of the interaction.