The iodide ion radius is much larger than the other common halides, which results in the negative charge being dispersed over a large space.

The industrial preparation of HI involves the reaction of I2 with hydrazine, which also yields nitrogen gas:[7] When performed in water, the HI must be distilled.

Another way HI may be prepared is by bubbling hydrogen sulfide steam through an aqueous solution of iodine, forming hydroiodic acid (which is distilled) and elemental sulfur (this is filtered):[8] Additionally, HI can be prepared by simply combining H2 and I2: This is a reversible reaction (using conditions 250°C) This method is usually employed to generate high-purity samples.

However, when a mixture of the gases is irradiated with the wavelength of light equal to the dissociation energy of I2, about 578 nm, the rate increases significantly.

In this method, I2 reacts with phosphorus to create phosphorus triiodide, which then reacts with water to form HI and phosphorous acid: Solutions of hydrogen iodide are easily oxidized by air: HI3 is dark brown in color, which makes aged solutions of HI often appear dark brown.

Like HBr and HCl, HI adds to alkenes:[11] HI is also used in organic chemistry to convert primary alcohols into alkyl iodides.

Although harsh by modern standards, HI was commonly employed as a reducing agent early on in the history of organic chemistry.