The zinc phosphating process in manganese-modified low-zinc phosphating baths has been monitored firstly by an in situ quartz crystal impedance system (QCIS) which allows rapid and simultaneous measurements of admittance spectra of the zinc-coated piezoelectric quartz crystal (PQC) resonator. The electrical equivalent circuit parameters are obtained by non-linear least square regression analysis of synchronously acquired conductance and susceptance data. The experimental results show that the kinetic growth process of zinc phosphate coating can be monitored on-line by measuring the equivalent circuit parameters and frequency shifts of the zinc-coated PQC resonator. According to the motional inductance (L-m) change of zinc-coated PQC during zinc phosphating treatment, the whole process can be divided into four regions: pickling reaction, exponential growth, linear growth and exponential decay growth, respectively. And effects of inorganic additives (Ni2+ and/or Mn2+) on the properties and performance of zinc phosphating coatings as well as the variations of equivalent circuit parameters of zinc-coated PQC during the phosphating treatment are also investigated.