Four days of industrial action have already taken place, with more action planned for 4 and 16 January.
Stagecoach said a "near-normal level of service" would operate in Barnsley, Rotherham and the Dearne Valley.
Paul Lynch, managing director of Stagecoach Yorkshire said: “I am very disappointed that we have not been able to reach an agreement after many months of negotiation. We have made several pay offers, all of which we believe would be seen as good offers by the public and one of which was actually recommended for acceptance by the union.
“It is therefore extremely disappointing that the union has chosen to call this strike which will cause unnecessary disruption to our customers and will also hit the pockets of many of our employees’ families.
“We will be working extremely hard to ensure that the majority of passengers are not affected by the strike action and have put together a robust contingency timetable. Our priority is to provide customers with a near normal service and we are drafting in more than 100 drivers from other areas, as well as additional managers, to make sure that any disruption is kept to a minimum.”
A special £1 flat single fare for adults will apply on these dates. Megarider tickets, child and pensioners free passes will be accepted as normal.