Apparatus and method for generating ruthenium tetraoxide in situ for use in vapor deposition, e.g., atomic layer deposition (ALD), of ruthenium-containing films on microelectronic device substrates. The ruthenium tetraoxide can be generated on demand by reaction of ruthenium or ruthenium dioxide with an oxic gas such as oxygen or ozone. In one implementation, ruthenium tetraoxide thus generated is utilized with a strontium organometallic precursor for atomic layer deposition of strontium ruthenate films of extremely high smoothness and purity.