The theory of reactive systems (RSs) represents a fruitful proposal for deriving labelled transition systems (LTSs) from unlabelled ones. The synthesis of an LTS allows for the use of standard techniques in the analysis of systems, as witnessed by the widespread adoption of behavioral semantics. Recent proposals addressed one of the main drawbacks of RSs, namely, its restriction to the analysis of ground (i.e., completely specified) systems. A still unresolved issue concerns the lack of a presentation via inference rules for the derived LTS, thus hindering the modularity of the presentation. Our paper considers open RSs. We first introduce a variant of the current proposal based on “luxes”: our technique is applicable to a larger number of case studies and, under some conditions, it synthesises a smaller LTS. Then, we illustrate how the LTS derived by using our approach can be equipped with a SOS-like presentation via an encoding into tile systems.