This paper analyses the regulatory context, i.e., how IFRS preparers investing in Poland react to banning IFRS use by domestic firms in the alternative trading market (NewConnect) after six years of regulatory arbitrage. In other words, it studies their portfolio investment and foreign direct investment (FDI) outward sensitivity to the end of regulatory arbitrage opportunity given by the alternative market’s regulation in the country where accounting law limits IFRS use to regulated markets and business groups where the parent company uses IFRS. The contribution is built on the natural experiment of prohibition of domestic IFRS preparers issuing shares on the alternative (not-regulated and cheaper) trading market after 2012. The originality is delivered by implementing a de facto measure of IFRS adoption by foreign investors to identify a 69% growth rate of IFRS preparers investing in Polish firms in 2012 (317% growth of Swedish investors and 219% of Portuguese) that dropped to 8% in 2013 and 3% in 2018. Thus, we check whether investing decisions consider opportunities to gain funds or attract other investors and build trust through IPOs on the unregulated market dedicated to start-ups and engaged in R&D activity. We use difference-in-differences, GMM dynamic panel-data analysis with Arellano-Bond and Arellano-Bover/Blundel-Bond estimators for the Knowledge-Capital model on portfolio investment, outward FDI overall, and three FDI components based on debt, equity, and earning reinvestment in 2003–2019. We found that banning IFRS use by domestic firms in the alternative market discourages portfolio investments, debt-based FDI flows and FDI earnings reinvestment. Also, total FDI and equity-based FDI from countries with more IFRS preparers have been reduced since 2013 due to the IFRS ban for domestic firms.