Ultrasonic time domain reflectometry (UTDR) was used to monitor the deposition of combined organic-colloidal fouling on a nanofiltration membrane. The fouling experiments were performed with different feed solutions: a mixture of 1000 mg/L silica and 1000 mg/L NaCl, a mixture of 1000 mg/L silica and 250 mg/L BSA, and 250 mg/L BSA alone, respectively. Results showed that the rate of flux decline obtained in the constant-pressure experiment with the mixture of silica + BSA was greater than that with the mixture of silica + NaCl and BSA alone. The acoustic measurements indicated that the fouling layer obtained from the combined organic-colloidal fouling was denser than that obtained from the colloidal fouling layer in the presence of NaCI. Furthermore, the mixed foulants rapidly deposited on the membrane surface in the early fouling phase, and then reached a plateau in the later fouling phase under a constant pressure operation, whereas the mixed foulants gradually deposited on the membrane surface as the fouling progressed under a constant flux operation. The main reason for the rapid foulant deposition during the initial fouling stage under constant pressure operation was because the initial flux was above the critical flux. The off-line AFM analysis and zeta potential measurements corroborated the ultrasonic measurements. (C) 2015 Elsevier B.V. All rights reserved.