Here is the DGII website which gives you all the info on all the taxes.
DGII - Direcci?n General de Impuestos Internos
For income tax specifically look up ISR - Impuesto Sobre la Renta
DGII - Direccion General de Impuestos Internos
This was recently amended & they have yet to update webpage. The baseline for tax-free income has risen to RD$26,334.75 per month which is a RD$2,147.84 increase over 2007. Up to RD$316,017 income per annum is tax exempt; from RD$316,017 to RD$474,024 pa is taxed at 15%; income between RD$474,024 and RD$658,367 will be taxed at RD$23,701 plus 20% of the income over RD$474,024. Income over RD$658,367 will be taxed RD$60,570, plus 25% of the amount over RD$658,367.