The nature of the pseudogap phase of cuprates remains a major puzzle 1,2 . One ofnew signatures is a large negative thermal Hall conductivity κ xy , which appears for dopings p below the pseudogap critical doping p*, but whose origin is as yet unknown 3 . Because this large κ xy is observed even in the undoped Mott insulator La 2 CuO 4 , it cannot come from charge carriers, these being localized at p = 0. Here we show that the thermal Hall conductivity of La 2 CuO 4 is roughly isotropic, being nearly the same for heat transport parallel and normal to the CuO 2 planes, i.e. κ zy (T) ≈ κ xy (T). This shows that the Hall response must come from phonons, these being the only heat carriers able to move as easily normal and parallel to the planes 4 . At p > p*, in both La 1.6-x Nd 0.4 Sr x CuO 4 and La 1.8-x Eu 0.2 Sr x CuO 4 with

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