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Many applications are suggested for Ti-MXene motivating strong interest in studies of Ti₃C₂T_x synthesis by solution-based methods. However, so far only ex situ studies of the synthesis are performed, mostly due to the difficulty of handling HF-based solutions. Here the first time-resolved in situ synchrotron radiation X-ray Diffraction study of MXene synthesis performed using a plastic capillary-size reaction cell directly in HF solution is reported. This study provides the first report on the structure of “pristine MXene” formed by Ti₃AlC₂ etching with LiF+HCl. The term “pristine” refers to the MXene structure found directly in HF solution. By comparing the interlayer distances of pristine MXene (≈13.5 Å), solvent-free Li-intercalated MXene (≈12.2 Å), and Li-free MXene (≈10.7 Å), it can be concluded that the width of “slit pores” formed by terminated MX layers during the Al etching does not exceed ≈3 Å. The width of these slit pores is a key factor for HF etching of Al within the interlayers. This space constraint explains the slow kinetics of MXene formation in HF-based synthesis methods. No intermediate phases are observed, suggesting that the crystalline MXene phase is formed by the simultaneous etching of Al and termination of Ti₃C₂ layers.