All countries face several issues while running the process of sustainable development&mdash / the absence of a uniform means of sourcing investment for sustainable development and the lack of a unified index for the evaluation of sustainable development. No doubt, ensuring sustainable development requires constant financial investments. Hence, it is essential to examine the investment sources for sustainable development at the country level and to comprehend if the current financial investment has a direct impact on the results of a country&rsquo / s sustainable development. The article aims at identifying the financing sources for sustainable development for each of the European Union (EU) countries and assessing their impact on each of the EU countries&rsquo / sustainable development, which is expressed as the Integrated Sustainable Development Index (ISDI). After the detailed analysis of investment sources for the sustainability of the EU countries, two sources of investment, assignation of budget and the EU structural funds, were selected, and ISDI calculation was applied for twenty-five of the EU member states for the period 2003&ndash / 2017. Correlation analysis (using SPSS software) helped to identify the strength of the connection and to select countries for the Johansen Cointegration Test (using Eviews software) in order to determine how variables interact. The results show that the combination of the assignation of budget and the EU structural funds has a positive impact on the coherence of five (Czech Republic, Denmark, Spain, Slovenia, and Austria) out of twenty-four countries.