This article considers the problems of ensuring the effective taxation of income from internationally mobile capital. The focus is on the income taxation of dividends, interest and capital gains derived from cross-border portfolio investments by private persons as investors. The basic question is whether, under the existing legal system, it is possible to tax international portfolio investments effectively. In particular, the purpose of the article is to examine the legal rules - the domestic legislation on tax enforcement and the international treaties on cooperation between tax administrations - for securing Finland's taxing rights in respect of cross-border investments.