A simultaneous measurement of the top-quark, W-boson, and neutrino masses is reported for t (t) over bar events selected in the dilepton final state from a data sample corresponding to an integrated luminosity of 5.0 fb(-1) collected by the CMS experiment in pp collisions at root s = 7 TeV. The analysis is based on endpoint determinations in kinematic distributions. When the neutrino and W-boson masses are constrained to their world-average values, a top-quark mass value of M-t = 173.9 +/- 0.9 (stat.)=(+ 1.7)(-2.1) (syst.) GeV is obtained. When such constraints are not used, the three particle masses are obtained in a simultaneous fit. In this unconstrained mode the study serves as a test of mass determination methods that may be used in beyond standard model physics scenarios where several masses in a decay chain may be unknown and undetected particles lead to underconstrained kinematics.