We discuss phase-difference measurements of two bosonic modes and propose an experimental realization of the phase-difference operator by Luis and Sánchez-Soto (1993 Phys. Rev. A 48 4702). We show that a measurement of a two-mode state will collapse any state into a phase-difference eigenstate. We also show that the phase-difference number-difference commutator can be expressed in the phase-difference distribution. Finally, we discuss the associated minimum uncertainty states.