With a pK_a of ∼6.98, the cell-permeant, dual-excitation ratiometric pH indicator BCECF, AM is ideal for measuring changes in the cytosolic pH of most cells. Intracellular pH measurements with BCECF are made by determining the pH-dependent ratio of emission intensity (detected at 535 nm) when the dye is excited at ∼490 nm versus the emission intensity when excited at its isobestic point of ∼440 nm.