Ethylene and propylene are key building blocks for plastics. They are produced by cracking ethane or naphtha at high temperatures in large cracker furnaces. The complex product mixture is separated in a cryogenic section at elevated pressures. The subsequent splitting and analytical monitoring of the H2, C1, C2, C3 and C4 fractions pose particular challenges. Classical continuous gas analyzers and process photometers are becoming an attractive alternative to GC and MS techniques as they are rugged, easy to operate, and can eliminate cross-interferences.