With a proton beam of Tp = 1.2 GeV incident on the deuterium pellet target of the WASA detector setup all three basic double-pionic fusion reactions have been measured simultaneously. By use of quasifree kinematics the energy range 2.3 GeV ≤ √s < 2.5 GeV could be covered, which just coincides with the energy region, where the ABC effect and its associated resonance structure has been observed. From the isospin decomposition we see that the resonance effect is solely in the isoscalar part of the reaction process, whereas the isovector part exhibits a monotonic smoothly rising energy dependence and no ABC effect.