Two types of forbidden transitions are identified in In1-xGaxAs/InP undoped quantum wells (QWs) with compressive strain by low-temperature (1.8 K) magneto-optical absorption. One of them is due to the interband transitions with different principal quantum numbers and is observable mainly in a low magnetic field and the other corresponds to P- and D-type exciton states and gets stronger as the magnetic field increases. By analyzing the forbidden transitions the in-plane effective masses of electrons (me,ρ*) and heavy holes (mh,ρ*) are simultaneously determined, together with the z-direction (growth-direction) ones (me,z*,mh,z*). The theoretically predicted relation among the electron effective masses [Sugawara etal, Phys. Rev. B 48, 8102 (1993)], me,Γ6≪me,ρ*≪me,z*, where me,Γ6 is the band-edge electron effective mass of bulk material, is therefore verified. The difference between the values of me,z* and me,ρ* is found to decrease as the strain in the QW drops. © 2003 American Institute of Physics.