This study introduces a novel methodology for measuring, exploring and describing the processes of party nationalization and localization. The key component of this approach is using electoral volatility as a baseline measure for computing party localization indices. For the purposes of this study, conventional definition of localization as non-uniform territorial political response (Caramani, 2004) is redefined as non-uniform, but spatially contingent responses of territorial units to national political forces. Using network analysis, the authors introduce quantitative approach for studying spatial and dynamical aspects of party systems and demonstrate its usefulness and applicability in cases of two post-communist systems: Serbia and Croatia. The results demonstrate that different components of party systems (new versus stable parties) exhibit different levels of nationalization and localization, which has been overlooked in previous studies. In line with previous research (Bochsler, 2010c; Golosov, 2016; Schakel, 2017), this study confirms that the Croatian party system is less nationalized than Serbian, but there are some similarities between the two countries in terms of the distribution of electoral volatility. In the discussion part of the study, limitations of new methodology are explained and directions for its further development are outlined.