Lithium-ion Batteries (LIBs) are characterized by high energy and power density and long life and are currently used in many applications from portable devices to energy storage systems. These features increase safety concerns, especially when these devices are subject to thermal, mechanical, or electrical abuse. Abuse can lead to exothermic reactions of cell components and with each other, causing a rapid increase in temperature, called Thermal Runaway (TR), and pressure. The response to abuse depends on the physical-chemical characteristics of Li-ion cells, such as chemical composition and State of Charge (SOC). To study the effect of chemical composition, three different 18650 Li-ion cells were tested, i.e., Lithium Titanate Oxide (LTO), Lithium Iron Phosphate (LFP) and Lithium Nickel Cobalt Aluminium Oxide (NCA), at the same SOC (100 %). The cells were subjected to thermal abuse tests in a tubular reactor connected at the output to an online Fourier-transform infrared spectroscopy (FT-IR). All events, i.e., Current Interrupt Device (CID) activation, venting and TR, were recorded, and the gases emitted were traced back to the reactions that take place inside the cell. By comparing the response of the cells with different composition it was found that onset of TR occurs at lower temperature for NCA than the other cells (207 vs 233-234 °C), but the maximum temperature reached during TR by the NCA is higher (579 vs 310-338 °C). Regarding toxic emissions, for all three cells the values of hydrofluoric acid (HF) and carbon monoxide (CO) significantly exceed the Immediately Dangerous to Life or Health Limit (IDHL) defined by the National Institute for Occupational Safety and Health set at 30 ppm for HF and 1200 ppm for CO in 30 min, with maximum concentration of HF between 824 - 893 ppm and the maximum concentration of CO changing according to the chemistries: 231990 ppm for NCA, 140728 ppm for LTO and 97140 ppm for LFP.