This is a submitted version of a paper published in Nonlinear Analysis.Citation for the published paper: Babaoglu, C., Bazarganzadeh, M. (2011 Abstract. In this paper, we investigate general properties of the two-phase quadrature domains, which recently has been introduced by EmamizadehPrajapat-Shahgholian. The concept, which is the generalization of the wellknown one-phase case, introduces substantial difficulties with interesting and even richer features than its one-phase counterpart. For given positive constants λ ± and two bounded and compactly supported measures µ ± , we investigate the uniqueness of the solution of the following free boundary problemwhere Ω = Ω + ∪ Ω − . It is further required that the supports of µ ± should be inside Ω ± ; this in general may fail and give rise to non-existence of solutions. Along the lines of various properties that we state and prove here, we also present several conjectures and open problems that we believe should be true.