A simultaneous measurement of the top-quark, W-boson, and neutrino masses is reported for tt¯ 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 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 Mt=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.