Composite ionic liquid (CIL) prepared from triethylamine hydrochloride, anhydrous aluminum(III) chloride, and cuprous chloride is a new catalyst for isobutane alkylation. This composite ionic liquid alkylation (CILA) technology yields an alkylate with favorable product distribution. CILA is a promising replacement for sulfuric acid and hydrofluoric acid alkylation technologies. However, some solids are formed during alkylation, and solid amount increases with time on stream. In a number of separation steps, the used CIL was divided into liquid and solid. Nuclear magnetic resonance, X-ray diffraction, X-ray photoelectron spectroscopy, and elemental analyses were applied to characterize these fractions. The content of acid-soluble oil (ASO) in solid was also investigated. The results showed that the solid was mainly cuprous chloride and contained about 1 wt % ASO. The loss of aluminum chloride and detachment of CuCl as a form of solid from CIL resulted in the decrease of CIL acidity and product selectivity.