Methods and systems for determining for determining asphaltene onset pressure of a formation fluid are described herein. The method includes the following processes: (a) transmitting light through a sample of the formation fluid; (b) decreasing pressure of the sample; (c) detecting intensity of the transmitted light during depressurization; (d) identifying a change in intensity of the transmitted light during depressurization; (e) increasing pressure of the sample to a fixed pressure; and (f) detecting intensity of the transmitted light at the fixed pressure and at an equilibrated light intensity. Processes (a) to (f) are repeated for a number of different fixed pressures. The asphaltene onset pressure of the formation fluid sample can be determined using (i) the intensity of the transmitted light during each depressurization and (ii) the intensity of the transmitted light at each of the different fixed pressures.