The Collaborative International Dictionary of English v.0.48:

Acidulous \A*cid"u*lous\, a. [L. acidulus, dim. of acidus. See
   Acid.]
   Slightly sour; sub-acid; sourish; as, an acidulous tincture.
   --E. Burke.
   [1913 Webster]

   Acidulous mineral waters, such as contain carbonic
      anhydride.
      [1913 Webster]

WordNet (r) 3.0 (2006):

acidulous
    adj 1: being sour to the taste [syn: acidic, acid,
           acidulent, acidulous]

Moby Thesaurus II by Grady Ward, 1.0:

64 Moby Thesaurus words for "acidulous":
   acerb, acerbate, acerbic, acetic, acetose, acetous, acid, acidic,
   acidulated, acidulent, acrid, acrimonious, astringent, biting,
   bitter, burning, caustic, choleric, corroding, corrosive, cutting,
   double-edged, dry, edged, embittered, escharotic, fierce, harsh,
   hyperacid, incisive, keen, mordacious, mordant, penetrating,
   piercing, piquant, poignant, pungent, rancorous, rankled,
   resentful, resenting, rigorous, rough, scathing, scorching, severe,
   sharp, sore, splenetic, stabbing, stewing, stinging, strident,
   stringent, subacid, subacidulous, tart, trenchant, vehement,
   violent, virulent, vitriolic, withering