In this article we present LLSR, an R package for the characterization of aqueous two-phase systems (ATPS). The scope of existing mathematical descriptions is ported to R and expanded with the implementation of a database of phase diagram parameters. The package applies nonlinear regression analysis to calculate the fitting parameters of well-known empirical equations-such as the ones described by Merchuk and Murugesan-using data obtained from articles published in high quality journals. The database is based on empirical data obtained from published manuscripts and provides a software package with the potential to reduce experimental effort and save resources throughout the development of an ATPS-based process or product. The package requires only minimal knowledge of R and can be installed directly from its console, and the results are easily exported for subsequent analyses. Detailed examples of usage and descriptions of the package capabilities, including built-in database search functions, are demonstrated by characterizing a poly(ethylene glycol) 2000/ammonium sulfate ATPS, the data of which is provided with the package. Additional online resources for the package are also described, including the procedure to allow users to contribute to LLSR in order to expand its database and capabilities.