The photophysical properties of two pairs of squaraine molecules where two oxygen atoms in a squaraine bridge are replaced with sulfur atoms are investigated. This substitution leads to an inversion of the lowest singlet π−π* electronic transition by an n−π* transition, effectively reducing the energy difference between singlet and triplet states and significantly increasing the intersystem crossing efficiency without the use of “heavy atoms”. Experimental results, in agreement with quantum chemical calculations, show near-unity values for triplet quantum yield and singlet oxygen generation quantum yield for sulfur-containing squaraines, which are potentially useful for two-photon photodynamic therapy.Keywords (keywords): squaraine dyes; intersystem crossing; triplet quantum yield; singlet oxygen generation; two-photon absorption; singlet−singlet and triplet−triplet absorption