Directive No 2019/2121 of the European Parliament and of the Council of 27 November 2019 substantially amended Directive No 2017/1132 on certain aspects of company law. The Polish legislator implemented new EU regulations on 15 September 2023, when the Act of 16 August 2023 amending the Code of Commercial Companies and certain other acts (Journal of Laws of 2023, item 1705) entered into force, constituting the largest amendment to the transformation provisions of the Code of Commercial Companies for many years. One of the new regulations concerns the so-called side-stream mergers, i.e. mergers involving companies being in such configurations that (1) one person holds, directly or indirectly, all the shares in the merging companies or (2) the members of the merging companies hold their securities or shares in the same proportions in all merging companies. The Polish legislator not only fulfilled the obligation arising from the directive to adopt provisions regarding such cross-border mergers (see Article 51615 § 1 and 2 CCC), but also, in order to prevent the so-called phenomenon of reverse discrimination, introduced similar regulations for domestic mergers. This article aims to critically analyse the latter.