The perennial geophytic herb, Pachycarpus acidostelma, stands upright at 400 mm high, with milky latex, a firmly established rootstock and a slender, tube shaped caudex with a woody base.

The stems are simple and somewhat rough to the touch. The leaves are egg-shaped and broad, growing to about 35–80 × 20–35 mm long. The leaf apex is blunt with a minor projecting point. The leaf base gradually tapers and is curved. The leaf margins are thick, firm, sometimes slightly curved, with fine hairs. The lower surface of the midrib is covered with rigid hairs. Pronounced veins occur on the lower leaf surface. The leaf stalk is very short, growing to about 3.0–6.5 mm long.

Fig. 2. A plant and leaves of Pachycarpus acidostelma.

The inflorescence grows in a flat-topped flower cluster (corymbose), arising from the end of the stem with buds growing outside the axils, about 4 per stem. Inflorescence stalks are free, with about 4 to 6 flowers growing to about 10–30 mm in diameter. Flower stalks are covered in short, soft hairs that grow to about 15–21 mm long.

The flowers of Pachycarpus acidostelma are pleasantly scented, light cream to pastel yellow, 13–17 × 23–25 mm. The calyx is slightly rough to the touch and bent downwards, 7.5–9 × 3.5–6.5 mm. The corolla is saucer-shaped when young, with tips bending downwards with age. The corolla lobes are egg-shaped, 9–12 × 9 mm. The gynostegium (a compound organ in milkweeds, formed by the merging of the filaments and the style) grows up to 3.9–4.8 mm tall. The corona is cream in colour, with a touch of yellow at the base, 1.4–3.2 × 1.7–2.4 mm and 2.5–2.7 mm tall. The part of the corona near the point of attachment to the corolla is broad with two curved appendages attached. The appendages are 2–2.7 × 0.7–1.9 mm, toothed and deltoid, tapering to a sharp point. A narrow indentation occurs on the surface and between the appendages.

The anther appendages are oblong and sharply pointed, about  2 × 1.7–2 mm. The anther wings are somewhat pronounced, about 2.4–3.4 × 1–1.4 mm. The pollinarium has a small mass of about 0.75–0.85 × 0.45 mm, on which pollinators attach to during the pollination process. The translator arm (a distinctive structure of the subfamily Asclepiadoideae that connects the pollinia to the gland) is narrowly winged and about 0.9–1.4 mm in diameter. The pollinium is compressed, oblong, extends along the axis and is about 1.35–1.8 × 0.65–0.9 mm in diameter. The head of the style is about 3.3–3.7 mm in diameter.

Fig. 3. Pachycarpus acidostelma flowers.

The fruits of Pachycarpus acidostelma are single, egg-shaped, swollen follicles, growing up to 90 × 34 mm on a drooping flower stalk, winged and pointed at the tip of the flower stalk.