The aim of the paper is to analyse and present the comparative characteristics of the manufacturing industry and the determinants of additional investment in long-lasting assets of companies in the manufacturing enterprises in Slovenia and Croatia. These are neighbouring countries with a common historical heritage and comparable economic development. A comparative analysis of the basic characteristics and movement indices of the relevant variables revealed that Slovenia’s manufacturing industry is more developed than Croatia’s and has a different structure in which the metalworking industry predominates. In Croatia, on the other hand, no industry dominates within the sector, with food and metal processing industries being the most represented. To examine the determinants of additional investment at the company level, a panel analysis was applied to a refined sample of all manufacturing companies in Slovenia and Croatia for the last five-year period, forming two models with several sub models. The working hypothesis based on previous research was confirmed - that the basic determinants of additional investment in long-lasting assets of companies in the manufacturing industry of Slovenia and Croatia are: business success, capitalization, firm size, debt and firm efficiency. The differences in the direction of the influence of each determinant compared to expectations due to the prevailing results of previous research are explained, as well as the observed differences in the values of the parameters for Slovenia and Croatia. The contribution of the research results lies in a better understanding of the impact of relevant determinants on additional investments as a prerequisite for the survival and development of the company.