We report on the magnetic, thermodynamic and optical properties of the quasi-one-dimensional quantum antiferromagnets TiOCl and TiOBr, which have been discussed as spin-Peierls compounds. The observed deviations from canonical spin-Peierls behavior, e.g. the existence of two distinct phase transitions, have been attributed previously to strong orbital fluctuations. This can be ruled out by our optical data of the orbital excitations. We show that the frustration of the interchain interactions in the bilayer structure gives rise to incommensurate order with a subsequent lock-in transition to a commensurate dimerized state. In this way, a single driving force, the spin-Peierls mechanism, induces two separate transitions.PACS numbers: 75.10. Jm, 75.40.Cx, 71.70.Ch Low-dimensional quantum spin systems exhibit a multitude of interesting phenomena. For instance a onedimensional (1D) S=1/2 chain coupled to the lattice may show a spin-Peierls transition to a non-magnetic, dimerized ground state. In recent years, detailed studies of the first inorganic spin-Peierls compound CuGeO 3 have deepened the understanding of spin-Peierls systems substantially [1]. Even richer physics is expected if the spins are coupled additionally to orbital or charge degrees of freedom. A prominent example is the complex behavior of NaV 2 O 5 , which arises from the interplay of spin dimerization, orbital order and charge order [1]. Recently, TiOCl and TiOBr have been discussed as spinPeierls compounds with strong orbital fluctuations [2-9], assuming a near degeneracy of the t 2g orbitals in these 3d 1 systems. Different quantities such as the magnetic susceptibility [2], the specific heat [9], ESR data [3] and NMR spectra [4] point towards the existence of two successive phase transitions, which clearly goes beyond a canonical spin-Peierls scenario in which a single secondorder phase transition is expected. The high transition temperatures of T c1 =67 K and T c2 =91 K found in TiOCl are fascinating in a spin-Peierls context.The structure of TiOX consists of 2D Ti-O bilayers within the ab plane which are well separated by X=Cl/Br ions [10]. Quasi-1D S=1/2 chains are formed due to the occupation of the d y 2 −z 2 orbital in the ground state (see below), giving rise to strong direct exchange between neighboring Ti sites along the b axis (y direction) and negligible coupling in the other directions. Accordingly, the magnetic susceptibility of TiOCl is well described at high temperatures by the 1D S=1/2 Heisenberg model with an exchange constant of J ≈ 676 K [2,3]. In the non-magnetic low-temperature phase, a doubling of the unit cell along the chain direction has been observed by x-ray measurements for both TiOCl [10] and TiOBr [11], supporting a spin-Peierls scenario. However, the following experimental facts are not expected in a canonical spin-Peierls system: (i) the existence of two successive phase transitions [2-4,9], (ii) the first-order character of the low-temperature phase transition [9][10][11], (iii) the observation of inequivalen...