R-O-R' + HI -> protonation of O -> then nucleophilic attack by I-. For simple ethers: excess HI gives both alkyl iodides. Limited HI: I- attacks the less hindered carbon (SN2). For mixed ethers with 3° group: SN1 mechanism — 3° carbocation forms, I- attacks it. Example: (CH3)3C-O-CH3 + HI -> (CH3)3CI + CH3OH (via SN1 on 3° carbon). With excess HI, CH3OH -> CH3I. Anisole (C6H5-O-CH3) + HI -> C6H5OH + CH3I (phenyl C-O bond is NOT cleaved — too strong sp2 C-O). Zeisel's method: Detects and quantifies -OCH3 groups. R-OCH3 + HI -> CH3I; CH3I + AgNO3 -> AgI (yellow ppt, quantified gravimetrically).