A method of operating a vacuum pump system, the method comprising the steps of: - operating a primary vacuum pump comprising a variable speed motor; - connecting at least two secondary vacuum pumps in parallel with said primary vacuum pump , each of the at least two secondary vacuum pumps comprising a motor - dividing the secondary vacuum pumps in groups, each group comprising at least one secondary vacuum pump ; - assigning a priority for each of said groups; whereby the method further comprises the steps of: - conducting a first measurement of the pressure (p1) measured at an inlet of the vacuum pump system ; - comparing the first measured inlet pressure (p1) with a predetermined pressure value (p0), and, if said measured inlet pressure (p1) is higher than said predetermined pressure value (p0), operating at least one secondary vacuum pump part of the group having the highest priority assigned to it; - conducting a second measurement of the inlet pressure (p1) at the inlet of the vacuum pump system ; - comparing the second measured inlet pressure (p1) with said predetermined pressure value (p0) and, if, said second measured inlet pressure (p1) is higher than the predetermined pressure value (p0), operating the at least one secondary vacuum pump of the group having a second highest priority assigned to it, whereby, the method further comprises the step of starting said secondary vacuum pumps at a first predetermined startup load (S startup,1 ) if it comprises a fixed speed motor, and/or starting said secondary vacuum pump at a second predetermined startup load (S startup,2 ), if said secondary vacuum pump comprises a variable speed motor. - operating a primary vacuum pump comprising a variable speed motor; - connecting at least two secondary vacuum pumps in parallel with said primary vacuum pump , each of the at least two secondary vacuum pumps comprising a motor - dividing the secondary vacuum pumps in groups, each group comprising at least one secondary vacuum pump ; - assigning a priority for each of said groups; - conducting a first measurement of the pressure (p1) measured at an inlet of the vacuum pump system ; - comparing the first measured inlet pressure (p1) with a predetermined pressure value (p0), and, if said measured inlet pressure (p1) is higher than said predetermined pressure value (p0), operating at least one secondary vacuum pump part of the group having the highest priority assigned to it; - conducting a second measurement of the inlet pressure (p1) at the inlet of the vacuum pump system ; - comparing the second measured inlet pressure (p1) with said predetermined pressure value (p0) and, if, said second measured inlet pressure (p1) is higher than the predetermined pressure value (p0), operating the at least one secondary vacuum pump of the group having a second highest priority assigned to it, whereby, the method further comprises the step of starting said secondary vacuum pumps at a first predetermined startup load (S startup,1 ) if it comprises a fixed speed motor, and/or starting said secondary vacuum pump at a second predetermined startup load (S startup,2 ), if said secondary vacuum pump comprises a variable speed motor. <EOS>