Hydrogen abundances (or H/He ratios) are hard to determine in stars cooler than the Sun because the Balmer lines, when visible at all, are formed largely in the chromosphere, while the bands of CH and NH are often strongly saturated and badly blended with atomic lines. A few stars (the hydrogen-deficient carbon or HdC stars) are known to be extremely hydrogen-deficient, as their G bands of CH are absent despite an overabundance of carbon. A means of detecting less extreme cases of hydrogen deficiency would improve our understanding of red giant evolution. Minor variations in hydrogen content may be expected as the result of the mixing of processed material to the surface, and more radical changes might result from a star's shedding its entire hydrogen-rich envelope, say in the course of binary-star evolution.