Homogeneous Liquid–Liquid Extraction (HLLE) provides a valuable opportunity for recycling Rare Earth Elements (REE) by circumventing the liquid–liquid interface and avoiding the problems linked to the high viscosity of ionic liquids (IL). For optimizing these processes with the ultimate goal of implementing them at the industrial scale, it is crucial to understand the mechanisms at play at various scales. In this study, we focused on the extraction of two REE elements, La(III) and Ce(III) with an HLLE system composed of the IL [Chol][TFSI], water and betaine as the extractant. We first established the phase diagram of [Chol][TFSI]/betaine/water using 1H nuclear magnetic resonance (NMR) spectroscopy. We then determined the efficiency and distribution ratio of the HLLE for different betaine concentrations. HLLE was modeled thermodynamically to access the most likely number of betaine molecules involved in the extraction process, which appears to be 2. Finally, we monitored in situ the extraction using X-ray Absorption Spectroscopy (XAS) at the K-edge of both lanthanum and cerium. It allowed us on one hand, to get some insight into the local environment around the REE, and on the other hand to measure the local concentration of REE at each position within the extraction system. In this latter process, we could evidence an accumulation of REE accumulates close to the liquid–liquid interface, a feature that was suggested by some recent simulations.