Experimental determination of the structure of<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msubsup><mml:mrow><mml:mi mathvariant="normal">H</mml:mi></mml:mrow><mml:mrow><mml:mn>3</mml:mn></mml:mrow><mml:mrow><mml:mo>+</mml:mo></mml:mrow></mml:msubsup></mml:mrow></mml:math>

Three different measurements on the structure of the ${\mathrm{H}}_{3}^{+}$ molecular ion are reported. The measurements all make use of a new technique---the foil-induced dissociation of a fast molecular-ion beam. It is shown that the structure is equilaterally triangular in shape. The most probable length of side of the triangle is determined by the three measurements to be 0.97 \ifmmode\pm\else\textpm\fi{} 0.03 \AA{}, 0.95 \ifmmode\pm\else\textpm\fi{} 0.06 \AA{}, and 1.2 \ifmmode\pm\else\textpm\fi{} 0.2 \AA{}, respectively.