The analysis presented in this thesis shows a search for a heavy scalar boson in the 2-Higgs-doublet-model (2HDM) framework, carried out in the h -> WW -> e v mu v decay channel. Under the assumption, that the light Higgs boson of the 2HDM coincides with the particle discovered with a mass of 125 GeV, the search is performed in the cos(b-a) vs. m_H plane using 20.3 fb^-1 of collision data recorded by the ATLAS detector in 2012 with a centre-of-mass energy of 8 TeV. Since no evidence for an additional heavy Higgs boson H can be found in the investigated mass range of 135 GeV to 1000 GeV, excluded phase space regions with 95% CL are given for Type-I and Type-II 2HDMs for different values of tan(b). In both models, large parts of the parameter space can be excluded for masses up to 1000 GeV.