It has been known for several decades that the brightness of some organic fluorophores can be increased by simply changing the solvent from light water (H2O) to heavy water (D2O). Brightness enhancement factors of two or more have been reported and the effect has been exploited in practical fluorescence applications, especially as a potentially general method to increase image brightness and resolution in cell microscopy. Many different photophysical processes have been proposed over the years to explain why fluorophore quenching in H2O is usually quite efficient, and why it is often decreased in D2O. Recently, Maillard and coworkers suggested that a universal mechanism for fluorescence quenching in water is Förster Resonance Energy Transfer (FRET) from the electronically excited fluorophore to high energy overtones and combination transitions of O-H vibrations in the region 700-900 nm. In this scenario, an experimental switch from H2O to D2O eliminates these high energy O-H vibrational transitions and thus removes the associated FRET quenching pathway. It has been hypothesized that FRET quenching by water is the primary reason for the heavy water florescence enhancement effect for long wavelength cyanine dyes. This hypothesis prompted us to study ICG and ICB as a complementary pair of cyanine dyes. Since ICG and ICB have the same number and types of functional groups, we reasoned that the switch from H2O to D2O should not alter the fluorescence quenching mechanism for only one of the dyes. Moreover, if FRET-based quenching is a major pathway as proposed by Maillard and coworkers, then the 100 nm longer absorption/emission wavelengths of ICG should produce two specific corollaries.