This paper describes a novel analytical method that applies a gaspotentiometric oxygen probe (GOP) for characterizing gasification behavior of solid fuels. On the basis of GOP signals a developed gasification model enables the determination of fuel-specific properties, like effective reaction rate constant and overall activation energy. For its experimental validation two coal cokes were converted with four different gasification agents in a lab-scale fluidized-bed reactor. The results obtained will be discussed and compared with literature data. As this advantageous in situ measurement technique is fast, inexpensive, and easy to handle, it makes the GOP a predestined tool for monitoring and controlling gasification processes.