SIPTU will begin a ballot of its members at Dublin Bus later today.
Drivers at the company are being asked to consider industrial action, after they rejected a Labour Court pay rise proposal of 8.25% over the next three years.
Dublin Bus drivers want to see their salaries increased in line with Luas drivers.
Last week, Dublin Bus members at the National Bus and Rail Union voted in favour of industrial action by a majority of 96%.
Dublin Bus last night released a statement saying it would not comment on the possibility of industrial action until the results of all remaining trade union votes have been completed.
The result of the SIPTU's ballot will be known on Friday afternoon.