Fourier-transform infrared spectroscopy with the attenuated total reflection method provides a fast, reliable technique to monitor the conversion of oil to fatty acid methyl esters (FAME) during biodiesel production. In this work, we employed a linear calibration method to monitor the generation of FAME in the transesterification of soybean oil and methanol, catalyzed by immobilized Candida antarctica lipase B (CALB). The interaction of the immobilized CALB catalyst with the substrates and products improved its catalytic performance. The rate law changed from zero-order in the first run to quasi-first-order in the subsequent three reaction runs. The quasi-first-order rate law was characterized by a phenomenological compressed exponential function.