The interaction between bacteriophage T4 DNA and cationic gemini surfactants was studied by the use of fluorescence microscopy. Upon addition of surfactant, DNA undergoes a transition from random coil to globule, with an intermediate coexistence region. The state behavior of a DNA-gemini surfactant system was found to depend on spacer length, valency, head group size, and tail length. A series of alkanediyl-alpha,omega-bis-(dimethylalkylammonium bromide) surfactants with fixed tail length and variable spacer length s showed a minimum of compaction efficiency at s = 6 due to the competition between entropy loss and enthalpy gain. This occurs at roughly the same spacer length at which the critical micellization concentration shows a maximal value (at s = 5). In comparison with a single-tailed divalent surfactant (12-3-1) it was shown that the two-tailed equivalent (12-3-12) was more efficient in compacting DNA. A series of gemini surfactants based on cationic peptides with a alpha,omega-diamino alkyl spacer showed similar behavior upon changing the spacer length. Additionally, two surfactants based on diastereomers of tartaric acid with hexadecanoic acid tails and alpha,omega-diaminopropanyl and spermidine head groups, respectively, showed effects of head group size that depended strongly on entropy effects. The dependence on valency of the head group is found to be similar to what is known for mono- and multivalent ions, the latter being more efficient per unit of charge.