A new approach to the use of micelles in the fluorescent sensing of metal cations is proposed and applied to the case of Hg(2+). We demonstrate how it is possible to transform a system from an ON-OFF to an OFF-ON sensor by changing the length of the chain used to lipophilise a ligand that resides inside TritonX-100 micelles together with pyrene as the fluorophore. Three tetrathia-monoaza macrocyclic ligands have been synthesised with the same ring but functionalised on the nitrogen atom with a methyl (C1-NS4), an n-butyl (C4-NS4) or an n-dodecyl (C12-NS4) chain. The three ligands have been fully characterised in water containing TritonX-100 micelles by means of potentiometric titrations and their apparent protonation and complexation constants with Hg(2+) were determined. On the basis of the distribution diagrams obtained, the more lipophilic C12-NS4 has been developed as an ON-OFF fluorescent sensor for mercury: working at pH<4, in the absence of Hg(2+) the ligand is inside the micelles, protonated and non-quenching, while on addition of mercury the [C12-NS4Hg](2+) complex forms which remains inside the micelles and is quenching. On the other hand, the ligand of intermediate chain length, C4-NS4, can be used to obtain an OFF-ON sensor at 7.0<pH<9.5. In the absence of added metal at pH>7.0 the ligand is unprotonated, it stays inside the micelles and is quenching, while addition of Hg(2+) in the 7.0-9.5 pH range results in the formation of [C4-NS4Hg](2+), which is hydrophilic enough to leave the micelles and to be released into the bulk solution where it is no longer capable of quenching pyrene fluorescence. Additional studies on C1-NS4, C3-NS4 and C8-NS4 indicate that the optimal chain length to observe this OFF-ON behaviour is C(3)-C(4).