There is great disagreement between current evaluations of the role of bremsstrahlung and line excitation by electron impact and recombination in cometary X rays. To study the problem in detail, we use the exact values of the Roentgen satellite (ROSAT) high-resolution imager and wide-field camera instrument response functions and apply them to all nine exposure intervals in the ROSAT observation of comet Hyakutake to determine X ray two-parameter power law and thermal bremsstrahlung spectra. Thermal bremsstrahlung is a convenient form for two-parameter spectrum and not a physical reality in comets. Then spectral index is &agr;=1.97¿0.18≈2, spectral temperature is kT=360 eV, and the exposure-mean photon emission is 3¿1025 ph s-1 and 1.3¿1016 ergs s-1 in a range of 90--2000 eV and in an aperture radius of 1.2¿105 km. The exposure-mean ROSAT spectrum is in excellent agreement with the Extreme Ultraviolet Explorer (EUVE) observation of 7.5¿1024 ph s-1 in a range of 100--165 eV from comet Hyakutake. Then we follow the same methods as Northrop et al. [1997> to calculate a volume emission rate at the comet brightness maximum and to calculate that from bremsstrahlung with the Vega 2 electron flux. Yet we obtain a much lower emission than Northrop et al., at a level of 0.5% of the observed emission in accordance with results of Krasnopolsky [1996, 1997a> and Bingham et al. [1997>. Electron impacts of molecular, atomic, and ion species in comets produce mostly excitation of the X ray lines C 277 eV and O 525 eV at a level of 2% of the total X ray emission from comet Hyakutake. This value is calculated for the Vega 2 electron flux. The value for the Giotto electron flux is 3¿10-6 of the measured emission. Processes which are responsible for impurity radiation in fusion plasma either are inefficient or produce ultraviolet photons in cometary plasma, and the method of Bingham et al. is inapplicable to excitation of X ray lines in comets. The calculations show that the expected effects of bremsstrahlung and electron impact are very low in cometary X rays. The obtained spectral index &agr;=2 differs significantly from &agr;≈5 for scattering of solar X rays by attogram dust. These facts favor charge transfer of heavy solar wind ions as the dominant process of X ray excitation in comet Hyakutake. Charge transfer is not efficient in production of X rays on the Moon and other solid bodies. ¿ 1998 American Geophysical Union