Proton-proton correlations were observed for the two-proton decays of the ground states of 19 Mg and 16 Ne. The trajectories of the respective decay products, 17 Ne+p+p and 14 O+p+p, were measured by using a tracking technique with microstrip detectors. These data were used to reconstruct the angular correlations of fragments projected on planes transverse to the precursor momenta. The measured three-particle correlations reflect a genuine three-body decay mechanism and allowed us to obtain spectroscopic information on the precursors with valence protons in the sd shell.PACS numbers: 21.10.-k; 21.45.+v; 23.50.+z The recently discovered two-proton (2p) radioactivity is a specific type of genuine three-particle nuclear decays. It occurs when a resonance in any pair of fragments is located at higher energies than in the initial three-body (p+p+"core") nucleus, and thus simultaneous emission of two protons is the only decay channel. Three-body systems have more degrees of freedom in comparison with two-body systems, hence additional observables appear. In the case of 2p emission, the energy spectra of single protons become continuous, and proton-proton (p-p) correlations are available, which makes them a prospective probe for nuclear structure or/and the decay mechanism. For example, the first p-p correlations observed in the 2p radioactivity of 94m Ag have revealed strong proton yields either in the same or opposite directions which called for a theory of 2p emission from deformed nuclei [1]. Two-proton emission can also occur from short-lived nuclear resonances or excited states (see, e.g., [2,3,4]). Though in this case the mechanism of 2p emission may depend on the reaction populating the parent state, such nuclei can be easily studied in-flight. E.g., the cases of 6 Be [2, 5] and 16 Ne [6]were studied by analyzing their p-p correlations in the framework of a three-body partialwave analysis developed for three-particle decays of light nuclei. In particular, the study of 6 Be revealed the existence of three-particle p+p+α correlations [2] which matched the three-body components found theoretically in the p-shell structure of 6 Be [7]. Very recently, p-p correlations were also observed in 2p radioactivity of 45 Fe [8,9] where both the lifetime and p-p correlations were found to reflect the structure of pf -shell 2p precursors [9]. Such a way of obtaining spectroscopic information is a novel feature compared to studies of two-particle decays.In the present paper, we study for the first time the p-p correlations in sd shell nuclei via examples of the 2p decays of 19 Mg and 16 Ne. These nuclei with very different half lives (T 1/2 ≈ 4 · 10 −9 s [10] and T 1/2 ≈ 4 · 10 −19 s [11], respectively) and presumably different spectroscopic properties may serve as reference cases illuminating the nuclear structure of other possible 2p emitters with sdwave configuration.The decay properties of the 16 Ne and 19 Mg ground states and the related resonances in 15 F and 18 Na are shown in Fig. 1 which compiles the data from Refs....