We demonstrate a silicon-organic hybrid (SOH) Mach-Zehnder modulator (MZM) generating four-level amplitude shift keying (4ASK) signals at symbol rates of up to 64 GBd both at room temperature and at an elevated temperature of 80°C. The measured line rate of 128 Gbit/s corresponds to the highest value demonstrated for silicon-based MZM so far. We report bit error ratios of 10−10 (64 GBd BPSK), 10−5 (36 GBd 4ASK), and 4 × 10−3 (64 GBd 4ASK) at room temperature. At 80 °C, the respective bit error ratios are 10−10, 10−4, and 1.3 × 10−2. The high-temperature experiments were performed in regular oxygen-rich ambient atmosphere.